1. Identify compound (A)

When $\mathrm{NaCl}$ is heated with $\mathrm{K_2Cr_2O_7}$ and concentrated $\mathrm{H_2SO_4}$, chromyl chloride is formed:

$4NaCl + K_2Cr_2O_7 + 6H_2SO_4 \rightarrow 2CrO_2Cl_2 + 2KHSO_4 + 4NaHSO_4 + 3H_2O$

So,

$A = CrO_2Cl_2$

Number of atoms in one molecule of $A$:

$1(\mathrm{Cr}) + 2(\mathrm{O}) + 2(\mathrm{Cl}) = 5$

2. Identify compound (B)

Chromyl chloride reacts with $\mathrm{NaOH}$ to give sodium chromate:

$CrO_2Cl_2 + 8NaOH \rightarrow Na_2CrO_4 + 2NaCl + 4H_2O$

So,

$B = Na_2CrO_4$

Number of atoms in one formula unit of $B$:

$2(\mathrm{Na}) + 1(\mathrm{Cr}) + 4(\mathrm{O}) = 7$

3. Identify compound (C)

Acidification of chromate with dilute $\mathrm{H_2SO_4}$ in presence of $\mathrm{H_2O_2}$ gives blue peroxo compound $\mathrm{CrO_5}$:

$Na_2CrO_4 + H_2SO_4 + H_2O_2 \rightarrow CrO_5 + \text{side products}$

So,

$C = CrO_5$

Number of atoms in one molecule of $C$:

$1(\mathrm{Cr}) + 5(\mathrm{O}) = 6$

4. Total number of atoms

$\text{Total} = 5 + 7 + 6 = 18$

5. Comparison with stored correct answer

Derived answer = $18$

Stored correct answer = $18$

They match.