The correct option is
C Our leaders are elected by us.
A sentence is in active or passive voice depending on whether its verb is in active or passive voice. A verb is in active voice when its form shows that the subject does something or some action. A verb is in passive voice when something is done to the subject. In option A, the subject is "We" and the subject performs the action "elect". It is in the active voice, and thus, incorrect.
In option B, the subject is "She" and the subject performs the action "has won". It is also in the active voice, and thus, incorrect.
In option C, the subject is "Our leaders". They "are elected" i.e., the action is performed on the subject. The doer of the action is "us". Hence this sentence is in the passive voice, and option C is correct.
In option D, the subject is "Elections" and the action ("happen") is performed by the subject. It is in the active voice, and thus, incorrect.