In the following sentence, which word is a gerund?

sleeping is an essential part of maintaining the body's health.
health
maintaining
essential
sleeping

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.

the answer is subjective. i personally agree with the first answer for this question.

if one isn't strictly talking about objects then the correct answer would be 'my eyes' since according to this riddle i am asleep and in order to answer/open the door i need to open my eyes first of all.

however, if one is specifically looking for an object, then the first thing i would open would be the door

hope this !