The correct answer is A. PQRS.
The sentence is about how a child develops best, and it compares a child to a young plant. The first part of the sentence, “when a child develops best,” is the topic of the sentence. The second part of the sentence, “like a young plant,” is a simile that compares a child to a young plant. The third part of the sentence, “he is left undisturbed in the same soil,” is the explanation of how a child develops best.
The other options are incorrect because they do not follow the correct sequence of the sentence. Option B, QPRS, puts the explanation of how a child develops best at the beginning of the sentence. Option C, QPSR, puts the simile that compares a child to a young plant at the beginning of the sentence. Option D, PQSR, puts the topic of the sentence at the end of the sentence.