The correct answer is: D. No error
The sentence is grammatically correct. The word “to” is a preposition that is used to indicate the purpose of an action. In this sentence, the principal is asking his old pupils to take an interest in the school after he has retired. The word “after” is a subordinating conjunction that indicates that the action of the principal asking his old pupils to take an interest in the school will happen after he has retired.
The sentence is also clear and concise. The meaning of the sentence is easy to understand, and the sentence is not too long or too short.
The sentence is also relevant to the topic of the question, which is to find out whether there is any grammatical error in the sentence. The sentence does not contain any grammatical errors.