I have the same problem when testing for Kaplan. After careful thought, I knew that the reason why I choose the wrong option is that I understand the 1st sentence wrongly.
"it" in the 1st sentence doesn't refer to State Constituion, but refer to Legislature. So you can get the evidence "State Constituion and Legislature allow improvment of package" and conclusion "Only govenor doesn't support the new pension program". The assumption is surely 3.
Thanks for your explanation. But I do think 'it' represents State Constitution. Now I am clear. The argument gives us clue that the only obstacle of new program is the possibility that governor will probably veto the program, so there must be a premise that State Constitution will support the program, or the program will definitely be fail. Am I right?