Explanation: Just to elaborate a little on the answer, it can also be added that the pronoun her is the objective form of she—not the possessive—and it could have been used if the sentence had read: "I saw Martha and her dancing," for instance. In addition, hers—which is the possessive form of she—could have been used if the sentence had read: "I saw Martha dancing. Hers is a very unconventional style of dancing," just to provide you with an example. Therefore, the pronoun she is the one that correctly completes the sentence, since it is one of the subjects of the verb.
