Why Can Presidents Not Rely on Their Own Parties as a Reliable

Why can presidents not rely on their own parties as a reliable tool in policymaking, even if they are still valuable?

A) Presidents do not control members of their own party in Congress.To the contrary, members of Congress have considerable autonomy.
B) Members of Congress generally believe that their electoral prospects are unrelated to presidential success or popularity.
C) Congressional leaders often try to sabotage their own party's presidents so they can defeat them in the next presidential primary contest.
D) The parties are unpopular with the public at large, with increasing numbers of voters describing themselves as independents.