An adverb is a word or set of words that modify verbs, adjectives, or other adverbs. They answer questions such as how, when, where, why, how often, how much etc.
Option A) highly means to a high degree or more likely
Option B) heavily means to a great degree
Option C) regularly means recurring at constant intervals
Option D) quickly means at a very fast speed
The given sentence mentions 'it is good to exercise'. From the options given, the word that fits aptly is option C) regularly and the complete sentence is: It is good to exercise regularly. The other three options do not complete the sentence meaningfully are so are incorrect options.