The formula of calcium nitrate is $Ca(NO_3)_2$
The formula of mass or molecular mass =At mass of $Ca+2\times $At mass of N+$6\times$ At mass of oxygen
$\Rightarrow 40+2\times 14+6\times 16$
$\Rightarrow 164$
$\therefore$ % of N=$\large\frac{28}{164}$$\times 100=17$
Hence (a) is the correct answer.