Solution:Concept- Molecular formula is a way to represent a molecule by writing its number of atoms of each element present in its structure.
Mercury oxide is an ionic compound with mercury ion () as the cation and oxide ion () as the anion.
As per the crisscross method, while writing the ionic formula of any compound, the numerical value of each of the ionic charges crosses over to become the subscript of the other ion.
Thus, the subscript of becomes 2 and the subscript of becomes 2 and the molecular formula of calcium hydride is . This can be simplified by keeping the ratio of atoms same in the formula as .
Hence, the correct answer is 1.