You should . . . . . . . . exercise daily.

In the given sentence "You should take exercise daily," the most suitable word to fill in the blank is "take."

The verb "take" is in the base form, and when used with "should," it forms a modal verb construction, indicating advice or recommendation. In this context, the sentence is suggesting that the person should make it a habit to exercise daily.

The other options do not use the correct verb forms for the given sentence. Option A, "took," is in the simple past tense, which does not match the context of giving advice or making a general recommendation. Option B, "have taken," is in the present perfect tense, which is not appropriate for expressing a general or habitual action. Option D, "taking," is in the present participle form, which is also not suitable for conveying a piece of advice or recommendation.

Therefore, the correct answer is "C. take.