The correct option is B I don’t agree to what Mr. Singh said.
Noun Clause is often connected with the subordinate conjunctions from 'wh' family according to the context.
Option A is incorrect as its incomplete.
Option B is correct, as 'what' gives the answer to the sentence " I don’t agree to it"
Option C is incorrect as wrong preposition is used with 'agree'.
Option D is incorrect as wrong conjunction is used.