Question

How is the entire noun clause used in the sentence?
We had not agreed on where we would meet.
a. subject
b. direct object
c. predicate nominative
d. object of a preposition

Answer 1

a. subject

where we would meet

Example:
This year's fireworks show must have been expensive, which is an annual holiday tradition.

This phrase could have been
This year's fireworks show, which is an annual holiday tradition, must have been expensive.

A clause is segment in a sentence that can stand on its own since it has its own subject and predicate.
Noun clause starts with a interrogative words like where what and etc.
Adjective clause is one type of clause that describes a noun or pronoun in the sentence
Another is, adverb clause.

Answer 2

d).  we had not agreed on "it" (noun clause can always be replaced by "it")
on is a preposition, therefore, on it is a prepositional phrase.