The following sentence has been divided into parts. One of them may contain an error. Select the part that contains the error from the given options. If you don’t find any error, mark ‘No error’ as your answer. By the time I reaching / the metro station, / the train had departed.

The correct answer is By the time I reaching.

Explanation:
The given sentence is divided into three parts: "By the time I reaching / the metro station, / the train had departed."

1. The main clause of the sentence ("the train had departed") is in the past perfect tense, which indicates an action completed prior to a specific moment in the past.
2. The time clause introduced by "By the time" establishes that reference point in the past and requires a verb in the simple past tense.
3. The participle form "reaching" cannot act as the main finite verb of the clause. It must be replaced by the simple past tense verb reached.

Corrected Sentence:
"By the time I reached the metro station, the train had departed."

Therefore, the part containing the error is By the time I reaching.