$\text{ When }A=\varphi \text{ , then number of elements in }P\left(A\right)\text{ is }$….. here P(A) is set of all subsets of A

$\begin{array}{r}\mathrm{A}=\varphi \\ \Rightarrow n\left(A\right)=0\end{array}$

$\text{ Now, number of subsets of any set containing ' }n\text{ ' elements is }{2}^n\text{ . }$

$\begin{array}{c}\text{ Here, }n=0\\ \Rightarrow \text{ Number of subsets }=2^0\\ =1\end{array}$