The correct answer is C. don’t go.
The sentence is about a habit that the speaker has on Mondays. The speaker does not go home for lunch on Mondays, but instead has lunch in the canteen. This means that the speaker’s habit is to not go home for lunch on Mondays.
The other options are incorrect because they do not describe the speaker’s habit. Option A, didn’t go, is in the past tense, but the sentence is about a current habit. Option B, was going, is also in the past tense, and it implies that the speaker used to go home for lunch on Mondays, but no longer does. Option D, doesn’t go, is the correct answer because it is in the present tense and it describes the speaker’s current habit.