The correct option is C Let's not
Let’s examine the sentence: “____ waste any more time on this project.” The sentence talks about ‘wasting’ the time and so we can assume the blank would have a negative word to suggest ‘not to waste time.’ This rues out Option A and C as they do not contain a negative word. Option B cannot be used as questions start with ‘don’t’ and this sentence is a statement and not a question. Hence, Option D is the correct answer. It is a contraction of 'let us not.'