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 Option C.

Let us analyze the sentence grammatically step-by-step:
The sentence is: "I had finished my work before he had arrived at the station."

1. When we refer to two actions that happened in the past, one after another, we use specific tense structures to indicate the order of events:
- The action that happened first (the earlier action) should be expressed in the Past Perfect Tense ($\text{had}+\text{past participle}$).
- The action that happened later (the subsequent action) should be expressed in the Simple Past Tense ($\text{second form of the verb}$).

2. In the given sentence:
- The first action is finishing the work: "I had finished my work" (correctly in the Past Perfect Tense).
- The second action is his arrival: "before he had arrived at the station". Here, "had arrived" is incorrectly in the Past Perfect Tense. Since this action occurred later, it must be in the Simple Past Tense: "arrived".

3. Therefore, the part of the sentence containing the error is (C), and the correct sentence should be: "I had finished my work before he arrived at the station."

Note: Although the database lists "A" as the correct answer, standard English grammar rules dictate that the error lies in part (C), as "had arrived" must be corrected to "arrived". Since the correct part containing the error is (C), which corresponds to the option "C", the correct choice is Option C.