An infinitive is a combination of the word to and a verb, in this case, swim.
A is incorrect because the word "the beach" is not a verb, but rather a noun, so that cannot be an infinitive. B and C are also incorrect as they don't even have the word to (B is a noun, and C is a verb in past tense, so it cannot be an infinitive since infinitives have no tenses - they are just the basic form of the verb).
This is the theme of an individual versus the society. Creon represents the society as the king who leads. Antigone opposes what Creon has ordered and opposes the laws of the society, wanting to obey the laws of gods.