The great hall is open to the public, you must reserve a spot in advance.

how is the comma used incorrectly in the sentence above?

the comma attempts to separate a compound predicate.
the comma attempts to join two independent clauses.
the comma attempts to set apart an essential element.
the comma attempts to separate a compound subject.

The comma attempts to join two independent clauses.

Explanation: Commas are not enough to join two independent clauses into a single sentence; the right punctuation mark for that function is the semicolon. All other answer options are incorrect because, in this case, the comma is not separating any part of an independent clause.

this need to be done in 15 minutes kind of passed the 15 minutes by now sorry dude but i can list some of my thoughts down for you : (

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 !