The boiling point of heavy water is $\displaystyle 101.42^oC $ which is higher than the boiling point of normal water $\displaystyle (100^oC) $ due to its higher molar mass.

Heavy water is called so because, it is the oxide of heavy hydrogen $(D _2$ or $ _1H^2)$.

Option $A$ is not a deuterolysis reaction. It is not a chemical reaction at all. 

$D _2O$  is used as a moderator in atomic reactors and as coolant in atomic reactors.

$Mg _{ 3 }{ N } _{ 2 }+6{ D } _{ 2 }O \rightarrow 2{ ND } _{ 3 }+ 3Mg(OD) _{ 2 }$

Answer:-

Heavy water acts as a neutron moderator in nuclear reactors to slow down neutrons so that they are more likely to react with the fissile $U _{235}$ than with $U _{238}$, which captures neutrons without fission process.

As Deuterium contains one extra neutron, so heavy water has higher molecular mass than water and because of that it is less reactive.

Deuterio hydrate of magnesium sulphate is $MgSO _4.7D _2O$. So, 7 molecules of heavy water molecules is present.

Reaction of process:
$4D _{2}O+3Fe \xrightarrow{Red:hot}Fe _{3}O _{4}+ 4D _2$.

As molecular mass of heavy water is higher than water so its physiological action is different from ordinary water and because of that reason it can not be used as drinking water.

$D _{2}O-$ Heavy water

Freezing point of heavy water is $3.82^oC$. Heavy water is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium, rather than the common hydrogen-$1$ isotope that makes up most of the hydrogen in regular water.

As D contains one extra neutron so heavy water has higher molecular mass than water and its electrolysis takes more time.
Also because of that it is less reactive and bond dissociation energy for $D _{2}$ is greater than $H _{2}$.
And $D _{2}O$ freezes (3.8C) at higher temperature than $H _{2}O$

Calcium carbide reacts with ordinary water and acetylene forms, similarly calcium carbide reacts with heavy water and  deuterated acetylene $C _{2}D _{2}$ forms.
Reaction:

Heavy water is $D _2O$, it has higher molar mass than water.
It is used in nuclear reactors, where it acts as a neutron moderator to slow down neutrons so that they are more likely to react with the fissile uranium-235 than with uranium-238, which captures neutrons without fissioning.
Also, dielectric constant of $D _2O$ is lower than water so solubility of inorganic substances is greater in$H _{2}O$ than $D _{2}O$.

In atomic reactor, heavy water is used as coolant and moderator. As a coolant, it removes heat from the reactor and as a moderator, it slows down the speed of neutrons.

Hard water is water that has high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of calcium and magnesium-containing minerals such as limestone, chalk and dolomite.
The following processes will produce hard water
1. saturation of water with $CaCO _3$
2. saturation of water with $MgCO _3$
3. saturation of water with $CaSO _4$
Hence options A,B & D are correct.

The freezing point of water is $0 ^oC$, whereas the freezing point of heavy water is $3.8 ^0C$. 

The reaction of ethylmagnesium chloride with heavy water forms mono deuterated ethane. $CH _3CH _2MgCl + DOD \rightarrow CH _3CH _2D + MgClOD$.

Due to the light weight of protium than deuterium, a bond to a protium atom can be broken $18$ times faster than the bond to a deuterium atom.

Higher boiling point of water is due to hydrogen bonding. Several water molecules associate through H-bonding. Large amount of energy is required to break this association.

Permanent hardness is the hardness that cannot be removed by boiling. It is usually caused by the presence of calcium sulphate or magnesium sulphates or chlorides, nitrates of Ca, Mg in the water which does not precipitate out as the temperature increases.

$D _2O$ has a somewhat higher boiling point because its molecular weight is about $(16+2+2=20)$ and $H _2O$ is about $(16+1+1=18)$.

${ D } _{ 2 }O$ can exchange proton from $-OH$ group of given options.

Boiling point of heavy water is more than normal water and therefore, bond energy of $O-H$ bond in water is lesser than bond energy of $O-D$ bond in heavy water. 

$D _{2}O$ is prepared by repeated electrolysis of ordinary water which contains a small amount of alkali. Ordinary water contains a very very small fraction (about 1 part in 6000 parts) of heavy water.

The boiling point of heavy water is $101.4^{0}C $ or $374.4 K$. Its melting point is $3.8^{0}C $ or $276.8 K$.

Heavy water $(D _2O)$ is a compound containing deuterium and oxygen. It is used in the atomic nuclear reactor as a moderator. It differs from ordinary water $(H _2O)$ only because it has Deuterium i.e. a heavy isotope of hydrogen.

In nuclear reactors, heavy water is used as moderator for neutrons.

The amount of energy required to break $H-H$ bond of 1 mole of gaseous hydrogen is 436 $kJ \ mol^{-1}$. This is known as bond dissociation enthalpy.
$H _2{(g)} + 436 \  kJ mol^{-1} \rightarrow  H{(g)} + H{(g)}$

High concentration of heavy water retards the growth of the plants. Tobacco seeds do not grow in heavy water.

Deuterium oxide $D _{2}O$ is known as heavy water. When both $H$ atoms of water $(H _2O)$ are replaced with $D$ atoms, heavy water is obtained.

$CaC _{2}+2D _{2}o\rightarrow Ca(OD) _{2}+C _{2}D _{2}$

Heavy water is deuterium oxide $(D _2O)$. Ordinary water contains a fraction (1 in 6000 parts) of heavy water. On large scale, it is produced by repeated electrolysis of ordinary water containing alkali. It is also produced by repeated distillation and condensation. It is also obtained by exchange process.

Heavy metals is used as a moderator in nuclear reactors to bring the fission reaction by slowing down the neutrons and capturing them. The main reason is that the heavy water captures less neutrons than the normal water. Later it can be used as a fuel which has not undergone enrichment to increase the percentage of Uranium-235

Heavy water is used in nuclear reactors, where it acts as a neutron moderator to slow down neutrons so that they are more likely to react with the fissile uranium-235 than with uranium-238, which captures neutrons without fissioning.

Organic ion exchange resins are used for water softening. In this process, the $Ca^{+2}$ and $Mg^{+2}$ ions which cause hardness are exchanged for $Na^{+}$ or $K^{+}$ ions due to which the water becomes decalcified or hardness free. Therefore, organic ion exchange resins purify hard water to yield only decalcified water

Hardness of water can be detected with soap water. Water is described as being 'hard' if it does not lather readily with soap. 

Moderator. e.g. , Heavy water, is a substance used in nuclear reactors of reduce the speed of fast Neutrons produced by nuclear fission.Moderator. e.g. , Heavy water, is a substance used in nuclear reactors of reduce the speed of fast Neutrons produced by nuclear fission.Moderator. e.g. , Heavy water, is a substance used in nuclear reactors of reduce the speed of fast Neutrons produced by nuclear fission.'

Heavy water is used in nuclear reactors, where it acts as a neutron moderator to slow down neutrons so that they are more likely to react with the fissile $U^{235}$ than with $U^{238}$, which captures neutrons without fissioning.

Heavy water is $D _2O$ and it is composed of oxygen and heavier isotope of hydrogen $(D)$.

Reaction: