The correct answer is (4)
Sol. $r_{n} \propto \frac{1}{m}$
$
\begin{array}{l}
\mathrm{r}_{\mu}=\frac{0.51}{207}=2.56 \times 10^{-13} \mathrm{~m} \\
\mathrm{E} \propto \mathrm{m}_{\mathrm{e}} \\
\begin{array}{l}
(E)_{\mu} & =-13.6 \times 207 \\
& =-2.8 \mathrm{keV}
\end{array}
\end{array}
$