The correct option is
D swimming
The given sentence indicates that George likes to swim in the sea. Thus, the answer must indicate that very fact. Further, the sentence already contains a main verb. Thus, the answer must be a non-finite verb. Also, George's action of enjoying to swim in the sea is still there meaning that the answer must indicate a continuous action.
Option A is incorrect. In 'from swimming', 'from' means 'having a starting point from or at'. Since, swimming is an activity, it does not have a starting point.
Option B is incorrect. 'swim' is a verb. Having two main verbs in one sentence will make it grammatically incorrect.
Option C is incorrect. 'to swim' is a non-finite verb, although, it does not indicate a continuous action. Thus, it is incorrect.
Option D is correct. 'swimming' is a non-finite verb. Further, it also indicates a continuous action. Thus, it is the correct answer.