A pronoun is wrong on ACT English when it could point at more than one noun, or at no noun at all. Replace it with the noun, or rewrite the sentence. Agreement in number is a different check, in pronoun agreement.
Who is they?
When Maya talked to Jordan, she was upset is unclear. Name the person. This and which pointing at a whole idea instead of a noun are also usual misses. Case (who versus whom) is not this problem, in pronoun case.
Relative pronouns that pick the wrong kind of noun are in relative pronouns. Grammar overall is in ACT grammar rules.
The bottom line
If you cannot circle the one noun the pronoun replaces, the pronoun is wrong.
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