In C and D, cannot often carry out suggests that a President with limited time suffers only from an inability to achieve legislative goals frequently, not from a frequent inability to achieve them at all.
我的理解:
A "is often unable to carry" often 修饰 unable,表示总统想执行,但是往往不能执行了。
C D "can not often carry out" often 修饰 carry, 表示总统想经常执行,但是不能。