P. Thinking about it pollutes and undermines the attitudes needed for

Sentence Q tells us what most people think about failure and that they consider it as toxic waste. The pronoun ‘it’ used in the sentence P has been used for failure, hence statement P will follow the statement Q. Sentence S tells us about the author’s view on failure and is a contradictory one to statement Q, so S will follow P because QP is a mandatory pair. Sentence R further explains the views of the author on failure. Hence, option D is the correct answer.