In the following question, a sentence is divided into four parts. Choose the part of the sentence which may contain a grammatical error in it. If all the parts of the sentence are grammatically correct and contextually meaningful, choose option “No error” as the answer choice.
I had (A)/ finished my work (B)/ before he had arrived (C)/ at the station. (D)

The correct answer is A.

Step-by-step Explanation:
Let us analyze the sentence piece by piece: "I had (A)/ finished my work (B)/ before he had arrived (C)/ at the station. (D)"

1. Understanding the Sequence of Past Events:
When expressing two actions that occurred in the past, the rule of tense coordination dictates that the action which occurred first is expressed in the past perfect tense, while the action that occurred later is expressed in the simple past tense.

2. Analyzing the Sentence Parts:
- The first action (finishing the work) is written as "I had finished..." in parts (A) and (B).
- The second action (his arrival) is written as "...before he had arrived" in part (C).
- Using the past perfect auxiliary verb "had" in part (A) creates an incorrect grammatical structure when coordinating these past events with the conjunction "before". Therefore, the grammatical error lies in part (A).