Given below are two statements: one is labelled as Assertion (A)...

The radius of the $n^{\text{th}}$ Bohr orbit of a hydrogen-like species is given by:
$r_n = a_0 \dfrac{n^2}{Z}$
where $a_0$ is the Bohr radius of the hydrogen atom ($n=1, Z=1$).

For the Be$^{3+}$ ion, $Z = 4$. The radius of its second stationary orbit ($n = 2$) is:
$r = a_0 \dfrac{2^2}{4} = a_0 \dfrac{4}{4} = a_0$
Thus, the radius of the second orbit of Be$^{3+}$ is exactly equal to the Bohr radius of the hydrogen atom. Therefore, Assertion (A) is true.

The formula shows that the radius is directly proportional to the square of the principal quantum number ($n^2$) and inversely proportional to the atomic number ($Z$). Therefore, Reason (R) is false.

Hence, Assertion is true but Reason is false.

Answer: (A) is true but (R) is false