First Let me clearly mention that I belong to non English speaking country, so I may be wrong with my question

So, My brother encountered a question in his English test.

It is useless to me who ___ ill

The options were am/is/are and he answered is which is correct according to my knowledge. (It can be are too in other situations.) But according to the teacher answer should be am. I have no idea how it can be. He told that it was something related to relative pronoun anticident. I don't know much about it. So, I would like to request you to kindly provide with the correct answer with links to any kind of sources about these type of use.

^{(First let me clearly mention that I am from a non–English-speaking country, so I may be wrong with my question.)}

My brother encountered a question on his English test:

It is useless to me who ___ ill.

The options were am/is/are and he answered is — which is correct as far as I am aware. (It can be are, too, in other situations.)

But according to the teacher the right answer should be am. I have no idea how it can ever be am. He said that it was related to the relative-pronoun antecedent. I don't know much about it.

I'm asking what the correct answer really is, and if the right answer really is am, then for the explanation why it is correct, preferably with links to authoritative sources about these strange types of usage.