Refer to figure.
A hydraulic selector valve is a directional control device installed between the hydraulic supply and a double acting actuator. 

A double acting actuator has two hydraulic chambers separated by a piston: pressurising one chamber while connecting the other one to return moves the piston in one direction; reversing the connections moves it in the opposite direction. The role of the selector valve is precisely to perform this selection: depending on the position commanded by the pilot or the control unit, it routes system pressure to one side of the actuator piston while routing the opposite chamber to the return line, so that the actuator extends or retracts on demand. 

Typical examples include the landing gear selector (gear up or down) and the flap selector. 

Direct system pressure to either side of the piston of an actuator → CORRECT. This is the defining function of a selector valve: it routes hydraulic pressure to one chamber of a double acting actuator and connects the opposite chamber to the return, so that the piston moves in the commanded direction. The valve switches between the two flow configurations to extend or retract the actuator.

Discharge some hydraulic fluid if the system pressure is too high → INCORRECT. This is the function of a pressure relief valve, which protects the system against overpressure by releasing fluid back to the reservoir above a certain threshold. It is not the role of a selector valve.

Select the system to which the hydraulic pump should supply pressure → INCORRECT. Selecting between hydraulic systems supplied by a pump is the role of a transfer or priority valve, not a selector valve. A selector valve operates on a specific consumer, not on a system.

Automatically activate the hydraulic system → INCORRECT. Automatic activation of the hydraulic system is managed by pressure switches and control units that command the pumps. A selector valve does not activate any system; it simply routes pressure to one side or the other of an actuator on demand.