The correct option is C them stealing
The context refers to three people. Also, the question uses the present participle of the verb see.
"Them to steal" in option A is incorrect as it uses the prepositional phrase "to steal". This is incorrect in the context.
"Him to steal" in option B is incorrect as there are three people.
"Them stealing" in option C uses the plural third person pronoun 'them' and the continuous form of verb + ing (which indicates the temporary action which may not be happening at the time of speaking). Thus, this is correct.
"Him stealing" in option D is incorrect as the context refers to two men and a woman.
Thus, option C is correct.