I'm trying to do an excercise. I have to chose which one is the INCORRECT answer.

Although human beings aren't able to percieve such changes, it is thought the the sensitive nervous systems of some animals COULD / HAVE TO / MUST be affected by them.

The answer is apparently "have to"

Now, I agree in that it "sounds wrong" although I can't understand in grammatical terms.

Grammaticallt it seems to be that each option is correct! "could" refering to possibility and "must to" and "have to" being used to express certainty ?

Of the three answers, only 'have to' seems wrong to me. The other two are possible, but 'must' is not very scientific.
See Have to Versus Must.

I gave an answer there.