Explanation

(c) Time period of a simple pendulum of length $l$, is given by
$T = 2\pi\sqrt{\frac{l}{g}}$ …(i)
Where, g is acceleration due to gravity.
When $g^{'} = \frac{g}{4}$,
New, time period is
$T' = 2\pi\sqrt{\frac{l}{\frac{g}{4}}}$ …(ii)
Dividing Eq. (ii) by Eq. (i), we get
$\frac{T'}{T} = \sqrt{\frac{g}{\frac{g}{4}}} = 2$
$\Rightarrow$ $T^{'} = 2\ T$
Hence, new time period becomes twice of the original value.