The college, which already offers 100 majors, is considering adding another one. This is the house that Jack built. I see what you mean. Use who to refer to humans, and that and which to refer to animals and things. Only who changes form according to pattern.
Some writers have difficulty deciding whether who or whom is correct.
In this sentence, who is incorrect. Any time you are unclear if it should be who or whom in an interrogative sentence, recast it as a declarative sentence to see which function who takes. You do believe who. In that sentence, who functions as an object and the objective form is whom. For this example, the objective form her is correct, meaning that whom is correct in the earlier sentence.