The correct answer is A. Compensate for line inductive reactance.
Series capacitors are used to compensate for the inductive reactance of long transmission lines. This is done by connecting the capacitor in series with the line, which creates a capacitive reactance that opposes the inductive reactance. This can help to improve the power factor of the line and reduce losses.
Line inductive reactance is caused by the current flowing through the line creating a magnetic field. This magnetic field then induces a voltage in the line, which opposes the current. This can cause problems with power transmission, as it can lead to voltage drop and power loss.
Series capacitors can help to reduce these problems by creating a capacitive reactance that opposes the inductive reactance. This can help to improve the power factor of the line and reduce losses.
The other options are incorrect. Option B is incorrect because series capacitors are used to compensate for inductive reactance, not capacitive reactance. Option C is incorrect because series capacitors do not improve line voltage. Option D is incorrect because series capacitors are used to compensate for line inductive reactance.