$Na^{+}$,$K^{+}$,$Mg^{+2}$ and $Ca^{+2}$ ions are concentrated in animals.

Option $(E)$ is correct.

Potassium hydroxide is a good conductor of an electric current in the molten (fused) state and in aqueous solution. When it is in solid state it cannot conduct electricity but it can conduct electricity in aqueous solution state or molten state. This is because it is an ionic compound. Ionic compounds are made of charged ions which are held tightly in solid state but become mobile in molten or aqueous solution state. The movement of these mobile ions between electrodes helps in conducting electricity.

Hypokalemia is when blood's potassium levels are too low. Potassium is an important electrolyte for nerve and muscle cell functioning, especially for muscle cells in the heart. Your kidneys control your body's potassium levels, allowing for excess potassium to leave the body through urine or sweat.

Plants need a small amount but high concentration of K for specific functions in the cytoplasmand a major portion (∼90%) of it is localized in vacuoles, where it acts as an osmoticum. Maintenance can be achieved by other cation like sodium too .

The daily requirement of sodium is $600 \ mg$.

As sodium accumulates, the body holds onto water to dilute the sodium. This increases both the amount of fluid surrounding cells and the volume of blood in the bloodstream. Increased blood volume means more work for the heart and more pressure on blood vessels. Over time, the extra work and pressure can stiffen blood vessels, leading to high blood pressure, heart attack, and stroke. It can also lead to heart failure.

In C4 plants, sodium is a micronutrient that aids in metabolism, specifically in regeneration of phosphoenolpyruvate (involved in the biosynthesis of various aromatic compounds, and in carbon fixation) and synthesis of chlorophyll.

The minimum requirement of potassium per day for an adult is $4700\ mg$.

Option $(A),\ (B)$ are correct.

Option $(A),\ (B)$ are correct.

Option $(B)$ is correct.

(A) $2Mn{ O } _4^-(pink)+16H^++5O _2^{2-}\rightarrow 2Mn^{2+}+8H _2O+5O _2$.
(B) $Na _2O _2+O _2\xrightarrow [300 atm]{> 450^oC} 2NaO _2 ({ O } _2^-$ contains on unpaired electron in $\pi^*$ MO).
(C) $2Na _2O\xrightarrow {400^oC} Na _2O _2+2Na$.
(D) Gives only $H _2O _2$.

${ Ca }^{ 2+ }$ is the ion helpful for controlling heart beat and muscle contraction.

Simple proteins bonded with a non-proteinic prosthetic group (acting as cofactor) are called conjugated proteins.
Examples include  Glycoprotein, Lipoprotein, and Phosphoprotein.

For fixing atmospheric nitrogen -nitrolim is used and further $CaCN _2+C$ is used as fertilizer.

$Mg$ is present in chlorophyll which is a green pigment and is used in photosynthesis in green plants.

Hence option B is correct.

Calcium carbide is also used for ripening fruit artificially in some countries. When calcium carbide comes in contact with moisture, it produces acetylene gas, which is quite similar in reaction to the natural ripening agent ethylene.

Holoenzyme = Apoenzyme+ Cofactor.

$Ca^{2+}$ ions are required for contraction of muscles in humans.

Mg binds to phosphate group in ATP thus making a complex that catalyses phosphate transfer. 

The chief component of the nacre that constitutes the pearl is aragonite $(CaCO _3)$. Nacre also contains a small amount of conchiolin, a hornlike organic substance (albuminoid) that is the main constituent of the mollusk's outer shell.