Which of the following theorems states that the current at one point in a circuit due to a voltage at a second point is the same as the current at the second point due to the same voltage at the first point?

While calculating Thevenin resistance (Rth) or Norton resistance (RN), independent constant current sources in the circuit are:

While calculating Thevenin resistance (Rth), constant current sources in the circuit are:

According to the Maximum Power Transfer Theorem, maximum power is delivered from a source to a load when the load resistance (RL) is equal to the internal resistance (r) of the source. At this specific condition, what is the efficiency of the power transfer?