
Max M. answered 04/11/19
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Agree that it depends, both on the student, and on exactly what you think is tripping people up.
Its / it's, your / you're, and there / their / they're are all commonly confused, but I suspect that's more likely because people aren't thinking, or are rushing, or aren't editing their work, and not because they really don't know which word means what. In other words, I think if you asked most people directly what "your" vs. "you're" means, they would answer correctly, even though it's a very common mistake.
But for homophones that are actually confused with each other, one that I frequently see, even in edited writing, is compliment / complement.
That, and near-homophones affect / effect.