Graph of $\left|\Psi^{2}\right| v / s r$, touches $r$ axis at $1$ point so it has one radial node and since at $r=0$, it has some value so it should be for $' s^{\prime}$ orbital.
$\therefore n-\ell-1=1$
where $\ell=0 \Rightarrow n-1=1$
$\therefore n=2 \Rightarrow{ }' 2 s'$ orbital
