To be elected, how long must House members have been citizens of the United States? at least two years
at least six years
at least seven years
at least four years

The answer is 7.

At least seven years:

“No Person shall be a Representative who shall not have attained to the age of twenty five Years, and been seven Years a Citizen of the United States, and who shall not, when elected, be an Inhabitant of that State in which he shall be chosen.”