Which one the following statement regarding helium is incorrect?

Option (A) is incorrect because helium is not lighter than hydrogen.

Hydrogen ($H_2$) has a molar mass of $2\ \text{g mol}^{-1}$, whereas helium (He) has a molar mass of $4\ \text{g mol}^{-1}$. Therefore, hydrogen is lighter than helium.

Helium is used to fill balloons and airships not because it is lighter, but because it is non-inflammable and chemically inert, making it much safer than hydrogen.

Option (B) is correct. Helium is used as a coolant in gas-cooled nuclear reactors due to its inert nature and high thermal conductivity.

Option (C) is correct. Liquid helium is used for cooling superconducting magnets employed in instruments such as MRI and NMR systems.

Option (D) is correct. Because liquid helium has an extremely low boiling point ($4.2\ \text{K}$), it is widely used as a cryogenic agent for experiments at very low temperatures.

Hence, the incorrect statement is Option (A).