According to the Goldgeier and Saunders reading, how have constraints on presidential authority in foreign policy eroded and what factors have caused this erosion?

Congress, the Bureaucracy or inefficiency and the Allies were the actual restraints.

Explanation:

Presidential responsibility limits weakened due to:

Polarization (political and ideological gap, now dividing the normal democratic and authorized republics, major sides of the same coin) restricting bipartisanship with Congressional. Reducing perceived threat (if, for instance, an increase in China were ever to occur, Congress might eventually be able to compromise on activities).

Trump followed such changes in the policy of the US.

Therefore, it's the right answer.