The correct option is A I have nothing but respect for him.
We use but as an alternative to except (for), apart from and bar to introduce the only thing or person that the main part of the sentence does not include. It is often used after words such as everyone, nobody, anything, anywhere, all, no, none, any, every.
But is used as a preposition when we put a noun or pronoun after it.
Option A is correct. It means I have nothing except respect for him. The noun phrase 'respect for him', which also acts as the object of the preposition 'but', sits after it.
Option B is incorrect because the meaning of the sentence has been reversed as it seems that the speaker has no respect for the person he is referring to.
Options C and D are incorrect because the sentence constructions are flawed and don't retain the meaning as well.