Refer to figure.
AC generators (otherwise known as alternators) convert mechanical energy into an alternating current.

When working in parallel and before connecting, alternators must have equal:

• Frequency: Damage can occur when there is a difference in frequency, as one generator tries to slow down and the other tries to speed up.
• Phase sequence: At any point in time, the phases of both generators must be identical (see graph).
• Voltage: the voltage output of both generators must be equal before being paralleled.

Therefore the correct answer is: “Voltage, frequency and phase”