The correct option is
C I have been painting my walls.
Few words can be used as both nouns and verbs. When a word is used as an action word, it is called a verb. When a word is used as the main subject of the sentence, it becomes a noun.Here, we have to find out a sentence, where 'paint' is used as a verb. Hence, 'paint' should be the action in the sentence and not the subject/object.
Option A - 'I love painting' is a wrong choice. Here, the subject 'I' is doing an action 'love' for 'painting'. Here, 'painting' is the object and hence it is a noun. Thus option A is incorrect.
Option B - 'Painting is an art' is a wrong choice. As the sentence mainly describes the subject 'dancing', it is a noun. Hence, option B is incorrect.
Option D - 'I like painting as it gives me peace' is a wrong choice. Here, 'dancing' is an object, as the subject 'likes' (verb) it. Hence it is a noun. Thus option D is incorrect.
Option C - 'I have been painting my walls' is the right choice. Here, the subject 'I' has been 'painting (action)'. Hence, it is a verb. Thus option C is the correct answer.