As mentioned in the comments, this question is about completing the sentence with a noun clause.
Answer:
He is a liar and so you need not believe what he says.
Step-by-step explanation:
A noun clause is a dependent clause that functions as a noun in a sentence. That means it can function as subject, object of a verb or of a preposition, or a predicate nominative. Noun clauses begin with words such as what, whatever, when, which, how etc.
Having that in mind, I will provide a couple of possible answers:
- He is a liar and so you need not believe what he says.
- He is a liar and so you need not believe that he is truly sorry.