ANS:C - the open circuit voltage

When we need to be applied Thevenin we take the load out the circuit and consider two terminal a And B (Rth, Vth).

Rth = the total input impedance in the circuit and we can apply it in two cases.

case 1: if they do not have to depend on the source we turn off all independent source Voltage source (Short Circuit) Current source ( open circuit).

case 2: if has a depend on the source we consider Voc = 1 v between terminal a and b or io = 1 A.

Rth = Vo/io.
Vth => Voc between two terminal.