Concept:Relative pronouns link a clause to a noun, and the correct choice depends on whether the noun refers to a person or a thing.
Explanation:The sentence talks about "the point", which is a thing or an idea, not a person.
When a relative pronoun follows a preposition such as "on", we use "which" for things.
Let us examine the options: "whom" is used only for people, so it is incorrect here.
"What" does not serve as a relative pronoun after a preposition in this construction, so it is not suitable.
"Whose" indicates possession and cannot refer back to an object of the preposition "on" in this way.
Therefore, "on which we disagreed" creates a proper relative clause and gives the intended meaning.
It can be understood as "the point about which we disagreed was trivial".
Answer:A. which