The most active halogen is fluorine. This is due to the small size and high electronegativity of Fluorine.

Fluorine, a pale yellow gas, is the least dense and chemically the most active, displacing the other halogens from their compounds and even displacing oxygen from water. Chlorine, a yellow-green gas, is denser and less reactive than fluorine.

$Mg+{H _2SO _{4}} _{(dil)}\longrightarrow MgSO _4+H _2$

Metals react with water to form a metal hydroxide or metal oxide and hydrogen gas. Intensity of reaction depends upon its chemical reactivity.

Calcium is more reactive according to reactivity series and it will displace iron in any reaction.

Metals low in actively series are very unreactive. The oxides of these metals can be reduced to metals by heating alone.

Aluminium doesn't react with water because, a thin oxidise layer is formed on it. But it reacts with acid and displace Hydrogen in it. Because, The acid react with acidic layer and remove it from Aluminium.  

$Li$ is the strongest reducing agent as its reduction potential is low $(-3.05)$.

With progressive increase in atomic number, the reduction potential of halogens decreases, thus oxidising power also decreases. Hence, a halogen with lower atomic number will oxidise the halide ion of higher atomic number and therefore, will liberate them from their salt solution.
Hence, the reaction
${ Cl } _{ 2 }+2{ F }^{ - }\rightarrow 2{ Cl }^{ - }+{ F } _{ 2 }$
is not possible.

An activity series is a list of substances ranked in order of relative reactivity. The most active metals are at the top of the table and least reactive at the bottom of the table. In chemistry, a reactivity series (or activity series ) is an empirical, calculated, and structurally analytical progression of series of metals, arranged by their reactivity from highest to lowest.

The reactivity series is a series of metals, arranged in order of reactivity from highest to lowest. It is used to determine the products of single displacement reactions, whereby metal A will replace another metal B in a solution if A is higher in the series.

$2Na + ZnCl _2 \rightarrow 2NaCl + Zn$

Among given metal oxides, magnesium have very low reduction potential so it can not be reduced by normal reducing agents.

Gold is least reactive metal because of higher standard reduction potential value.

Magnesium can displace hydrogen from steam. Mg is better reducing agent than hydrogen as Mg les above hydrogen in the reactivity series.
Hence, the tendency for the reaction $\displaystyle Mg \rightarrow Mg^{2+} + 2e^-$ is greater than the tendency for the reaction $\displaystyle 2H^+ + 2e^- \rightarrow H _2$. Hence, Mg will be oxidised and $\displaystyle H^+$ will be reduced.

The activity series is a listing of the metals in decreasing order of their reactivity with hydrogen ion sources such as water and acids. In the reaction with hydrogen ion source the metal is oxidized to a metal ion and the hydrogen is reduced to hydrogen gas. They are also good reducing agents and poor oxidizing agents.

Silver is the metal which is present at the bottom of reactivity series below $Mg$ and $Cu$ metals so it cannot displace these metals from its solution. In this case, no reaction takes place by this piece of silver.

In order to prevent the spoilage of potato chips, they are packed in plastic bags in an atmosphere of $N _2$ to prevent from being rancid or oxidised. 
Nitrogen is added to the potato chip packet to add strength to the packaging. Normally aluminium cans are used for packaging beverages, these beverages are carbonated drinks and the carbon dioxide present in them provides the internal pressure in the can and thus gives internal strength to the aluminium can. But in non-carbonated drinks, there is no carbon dioxide present and so Nitrogen is added to it to give the required strength to the packing.

Since, the copper is present below the hydrogen in the activity series it cannot displace hydrogen from the sulphuric acid.

$Li$ and $Na$ both metals belong to the alkali metals.

Y has displaced X in reaction I. So, Y is more reactive than X.
Z has displaced Y is reaction II. So, Z is more reactive than Y. 

In activity series metals are arranged in decreasing order of reactivity. The metals present at top of the series are highly reactive while metals present at the bottom are relatively inert in nature.

Metals present on top of the reactivity series above the hydrogen can displace the metals present below them from their salt solution.

Iron can displace $Cu$ from its solution

$Fe(s)+CuSO _4(aq)\longrightarrow Cu(s)+FeSO _4(aq)$

Oxygen is the second most electronegative element in the periodic table and has electronegative value 3.5. So, it creates polarity that strongly attract the electron towards itself.

Metals in the middle of the activity series are iron, zinc, lead, copper etc. 

Metal higher in the activity series is said to be more metallic than another which is below and the metals at the top of the activity series are so reactive that they are never found in nature in free state.

Reactive metals can displace less reactive metals from their compounds in solution or molten form. All metals are not equally reactive. Their reactivity is different for different metals.

Carbon cannot reduce the oxides of sodium, magnesium, calcium, aluminium etc. to the respective metals. This is because these metals have more affinity for oxygen than carbon.

Reactivity of metal depend on the formation of positive ions by losing electron. Sodium have more tendency to form positive ion than copper. Therefore sodium is more reactive than copper.

Metals which lose electrons less readily than hydrogen are said to be less reactive than hydrogen. All metals placed below hydrogen in the reactivity series lose electrons less readily than metals placed above hydrogen.

The metals at the top of the activity series an so reactive that they are never found in nature as free elements.

In the reactivity series, metals placed at the top are more reactive like sodium, potassium, calcium, magnesium etc.

Mercury, silver, gold are placed below copper in the reactivity series. These are also less reactive than hydrogen.

Only those metals which are more reactive than hydrogen, displace hydrogen from acids to form salts and evolve hydrogen gas. Out of the given elements, only $Na$ is a metal which is more reactive than hydrogen.

Metal $A$ displaces metal $B$ from its solution. Hence, it is more reactive than $B$.

In the activity series of metal $Zn$ is above the $Cu$ metal. So it can easily reduce $Cu^{2+}$ in the solution.

$Z$ is more reactive. It replaced lead from its compound.

Food cans are coated with tin and not with zinc because zinc is above the tin in reactivity series means more reactive than tin and can react with food elements preserved in it.  

Potassium is the most reactive metal so placed at the top in the reactivity series. Gold is least reactive so, placed at the bottom of reactivity series.

An acid neutralizes a base when they react with each other and respective salt and water are formed. Metal oxides are basic in nature. Thus, when an acid reacts with a metal oxide both neutralize each other. In this reaction, respective salt and water are formed.

Acid + Metal Oxide $\rightarrow$ Salt + Water

zinc oxide + hydrochloric acid $\rightarrow$ zinc chloride + water

$ 2HCl+ZnO \rightarrow  ZnCl _2+H _2O $

Al will displace iron from solution.
$\displaystyle  2Al+Fe _2O _3 \rightarrow Al _2O _3+2Fe$

In first reaction, $X(s)+CrCl _2(aq)\longrightarrow \text{ color change }+\text{ Bubbles formed.}$

By using the reactivity series of metals, one can predict the products of displacement reactions . Each element in the reactivity series can be replaced from a compound by any of the elements above it. For ex: $Mg$ can replace $Zn$ from the solution. 

An activity series is a list of substances ranked in order of relative reactivity. For example, magnesium metal can knock hydrogen ions out of solution, so it is considered more reactive than elemental hydrogen.

In nature, Oxidation occurs more than reduction. carbon, Sulfur, Nitrogen etc. gas converted to their oxides. This is oxidation. But, we never heard that their oxides converting to elements. So, $reduction$ is $less$.

Since carbon can't remove oxygen from $ZO , Z $ is the most active metal.

This answer can be explained by Reduction potential values:

$H^+$ ions are reduced at platinum electrode prior to $Zn^{2+}$.

In a displacement reaction, a more reactive metal of the activity series can displace a less active one in its salt solution. Reactivity order is: 
$Mg> Zn> Cu> Ag.$
Thus, we see that Ag is least reactive and of the given four metals, Ag would be displaced from the solution of its salts by other three metals.

On adding aluminium(Al) to dilute hydrochloric acid(HCl), the following displacement reaction takes place:
$2Al(s)  +  6HCl(aq)  \rightarrow   2AlCl _{3}(aq)  +  3H _{2}(g)$
Aluminium being more reactive than hydrogen, displaces the latter in hydrochloric acid $(HCl)$ to form aluminium chloride($AlCl _{3}$).

Silver and platinum are less reactive than copper. Hence, they can't displace copper from its solution.

The reactivity series is a series in which metals are arranged in decreasing order of their reactivities.
$Fe<Zn<Mg<K$
In the reactivity series, the most active metal is at the left side of the series and the least active metal is at the right side of the series.

Zinc is present above iron and copper in the activity series so it can displace both iron and copper from the salt solution. but question mention about other 3 so answer is none of these.

There will be no change in colour as copper(Cu) is less reactive metal than zinc(Zn) and thus cannot displace the latter from $ZnSO _{4}$ solution.