Explanation

(a) As we know that,
$\ \ \ \ \ P = \frac{V^{2}}{R}$
$\Rightarrow R = \frac{V^{2}}{P}$
$\Rightarrow \ \ R \propto \frac{1}{P}$
Hence, the resistance of 25 W bulb is greater than the resistance of 100 W bulb.
Now, both the bulbs are joined in series so the current will be same.
So, heat produced by the bulbs
$H = i^{2}\ Rt\ \Rightarrow \ H \propto R$
So, the heat produced by the 25 W bulb is greater than the bulb of 100 W, because its resistance is more than that of 100 W bulb. Hence, 25 W bulb will glow brighter.