4. the number of senators would increase only if ?

1 another state joined the united states

2 congress passed a constitutional amendment

3 the population of the united states increased

4 congress voted for an increase in special session

The right answer is:

1 If another state joined the United States.

Explanation:

There are debates even today , as there were in the late 18th century, on the size, composition and other crucial issues relating to the Senate. The US Constitution says simply that "the Senate shall be composed of two senators from each state." If for example, Puerto Rico decided to join the Union as the 51st state of the USA, then the Senate would have to be enlarged with two senators who´d represent Puerto Rico. But of course, if there was a bipartisan agreement, there could be an amendment to increase the number of senators, too.