Answer:
What is element definition in the HTML specification?
Step-by-step explanation:
The question a user should ask when determining whether a closing tag is needed is "What is element definition in the HTML specification?"
This is because the specification of an element determines how the elements can be used in HTML in accordance with the rules such as distinguishing which elements can have a closing tag.
Hence, in this case, the correct answer is "What is element definition in the HTML specification?"