The correct answer is A. PQRS.
The sentence is about the lack of air and water on the moon, and how this makes it unlikely that life exists there. The first sentence, “we shall hardly expect to find life there,” is the main clause of the sentence. The second sentence, “knowing that there is neither air nor water on the moon,” is a subordinate clause that provides evidence to support the main clause. The third sentence, “on the moon,” is a prepositional phrase that modifies the noun “there.”
The sentence is most logically structured in the order PQRS. In this order, the main clause comes first, followed by the subordinate clause, and then the prepositional phrase. This order makes it clear that the main clause is the most important part of the sentence, and that the subordinate clause and prepositional phrase provide supporting evidence.
The other options are not as logical. In option B, QRSP, the subordinate clause comes before the main clause. This makes it difficult to understand the main point of the sentence. In option C, RSPQ, the prepositional phrase comes before the main clause. This also makes it difficult to understand the main point of the sentence. In option D, SRQP, the main clause and subordinate clause are reversed. This makes the sentence difficult to read and understand.