Answer: C) Present participle.
Explanation: present participle is the form of a verb, ending in -ing in English, which is used in forming continuous tenses (for example: he is eating), as a noun (for example: bad thinking), and as an adjective (for example: interesting facts). In the given sentence we can see an example of a present participle used to form a continuous tense: "he is swimming" so the correct answer is option C.
