c
(c) Here $i = \frac{{110}}{{20 \times {{10}^3} + R}}$
$V = iR$
$ \Rightarrow $ $5 = \left( {\frac{{110}}{{20 \times {{10}^3} + R}}} \right) \times 20 \times {10^3}$
$ \Rightarrow $ ${10^5} + 5R = 22 \times {10^5}$
$ \Rightarrow $ $R = 21 \times \frac{{{{10}^5}}}{5} = 420\,K\Omega $
