Answer
(a) decrease
(b) I
Work Step by Step
(a) We know that the resistivity of the tungsten filament increases with increasing temperature and as a result its resistance is also increased while the voltage remains constant. Since $P=\frac{V^2}{R}$, this shows that power consumed and resistance are inversely proportional. Hence, at constant voltage the power consumed is decreased because the resistance increases.
(b) The best explanation is (I) -- that is, the voltage is unchanged, and therefore an increase in resistance implies a reduced power, as we can see from $P=\frac{V^2}{R}$
