In an active voice sentence, the subject of the sentence performs the action. 

(A) The letter was posted only yesterday. (Passive voice: "The letter" is not performing the action of posting.)
(B) The cheque was handed to her. (Passive voice: "The cheque" is not performing the action of handing.)
(C) The station master hoisted the flag. (Active voice: "The station master" is the subject performing the action of hoisting.)
(D) The passengers were stationed in an inn. (Passive voice: "The passengers" are not performing the action of stationing themselves.)
(E) They opened this outlet only last year. (Active voice: "They" are the subject performing the action of opening.)

So, sentences (C) and (E) are in active voice because the subjects ("The station master" and "They") perform the actions in those sentences.