Gerald Ford had difficulty being an effective president because of what?A) he refused to keep Henry Kissinger on as secretary of state B) Americans...

The best answer of the ones given here is clearly C. 
There are certainly other reasons why Ford had such a hard time during his presidency
but none of these other reasons (other than C) is presented
here.

In the wake of Watergate, new limits had been put on
presidential power by Congress.  Congress did not want another president to be able to
run amuk the way Nixon had.  Congress at that time was controlled by Democrats, not
Republicans.  For these reasons, C is the best
answer.

However, a much more important reason for Ford's
lack of success is that he was too closely linked to Nixon in the public eye.  This made
it hard for him to gain support among the people.