When determining Thevenin’s resistance of a circuit

When determining Thevenin’s resistance of a circuit

(a) all sources must be open circuited
(b) all sources must be short circuited
(c) all voltage sources must be open circuited and all current sources must be short circuited
(d) all sources must be replaced by their internal resistances

Ans. (d) all sources must be replaced by their internal resistances