Assertion: An electric lamp connected in series with variable capacitor and AC source, its brightness increases with increase in capacitance.
Reason: Capacitive reactance decreases with increase in capacitance of capacitor.

Capacitance reactance $X_C=\frac{1}{\omega C}$.When capacitance (C) increases, the capacitive reactance decreases. Due to decreases in its values, the current in the circuit will increases$\left(I=\frac{E}{\sqrt{R^2+X_C^2}}\right)$ and hence brightness of source (or electric lamb) will also increases.