Explanation

(c)
Let $S = 1 + 10 + 10^{2} + ... + 10^{90}$
$= \frac{1 \bullet (10^{91} - 1)}{10 - 1} = \frac{\left( 10^{13} \right)^{7} - 1}{10^{13} - 1} \times \frac{10^{13} - 1}{10 - 1}$
$= \left\lbrack \left( 10^{13} \right)^{6} + \left( 10^{13} \right)^{5} + \left( 10^{13} \right)^{4} + ... + 1 \right\rbrack \times (10^{12} + 10^{11} + ... + )$
$\therefore\$It is the product of two integers and hence not prime.