Explanation

(c)
Given that, probability of success $p = \frac{1}{4}$ and probability of unsuccess $q = \frac{3}{4}$
$\therefore Mean = np$
and standard deviation $= \sqrt{variance}$
$\Rightarrow 3 = \sqrt{Variance}$
$\Rightarrow$ variance = 9
$\Rightarrow npq = 9$
$\Rightarrow n \bullet \frac{1}{4} \bullet \frac{3}{4} = 9$
$\Rightarrow n = \frac{9 \bullet 4 \bullet 4}{3}\ \Rightarrow n = 48$
$\therefore Mean = np = \frac{1}{4} \times 48 = 12$