a
Both assertion and reason are true and reason is the correct explanation of assertion.

Radius, \({r_n} = \frac{{{n^2}{h^2}}}{{4\pi {e^2}mZ}} = \frac{{{n^2}}}{Z} \times 0.529\,\mathop A\limits^o .{r_n}\)

For first orbit of \(H-\) atom

\(n=1\)

\({r_1} = \frac{{{{(1)}^2}}}{1} \times 0.529\,\mathop A\limits^o  = 0.529\,\mathop A\limits^o \)