Explanation

(c)
Since, $r,\ s,\ t$ are prime numbers
$\therefore$ Selection of $p$ and $q$ are as under
$p\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ q$ Number of ways
$r^{0}$ $r^{2}$ 1 way
$r^{1}$ $r^{2}$ 1 way
$r^{2}$ $r^{0},\ r^{1},\ r^{2}$ 3 way
$\therefore$ Total number of ways to select $r = 5$
$s^{0}$ $s^{4}$ 1 way
$s^{1}$ $s^{4}$ 1 way
$s^{2}$ $s^{4}$ 1 way
$s^{3}$ $s^{4}$ 1 way
$s^{4}$ $s^{0},\ s^{1},\ s^{2},\ s^{3},s^{4}$ 5 ways
$\therefore$ Total number of ways to select $s = 9$
Similarly, the number of ways to select $t = 5$
$\therefore$ Total number of ways$5 \times 9 \times 5 = 225$