The reason why it is "their" because, if you were to use his, or its the main thing is that it wouldn't flow or sound right. That is the firs sign that the answer isn't correct. The other way to tell is because "his" doesn't mean more than one. And the last thing is that "its" isn't a pronoun for living things, that would be for inanimate objects.
Members of the senate is 100 people 2 for each 50 states and the number of people in the house of representatives depends on the population of a certain state but as of today there are currently 435
