Answer: D) Sleeping.
Explanation: a gerund is a word that is derived from a verb but that functions as a noun (it acts like the subject or the direct object in a sentence), in English the gerunds end in "-ing". In the given sentence: "Sleeping is an essential part of maintaining the body's health" the gerund is "sleeping" because we can see that it has a verb form (ending in -ing) but it is acting like a noun, more specifically it is the subject of the sentence.
