Explanation: An antenna is resonant when its total inductive reactance ($X_L$) equals its total capacitive reactance ($X_C$). When you add a capacitor in series with an antenna, you are introducing additional capacitive reactance into the circuit. To maintain the condition for resonance ($X_L = X_C$), the inductive reactance provided by the antenna must increase to balance this new, higher total capacitive reactance. For any given antenna, its inductive reactance ($X_L$) increases as the operating frequency increases. Therefore, to achieve resonance with the added series capacitance, the antenna's resonant frequency must increase. In essence, the added capacitance makes the antenna appear "electrically shorter" than its physical length, shifting its resonance to a higher frequency.