The following sentence has been divided into parts. One of them may contain an error. Select the part that contains the error.

The officer was unaware (A)/ that his assistant had (B)/ already submitted the documents (C)/ before he reach the office. (D)/ No error

To identify the part of the sentence containing the error, let us analyze the sentence structure and tense agreement step-by-step.

1. Analyze the main clauses and temporal sequence:
The sentence describes two actions that happened in the past: the assistant submitting the documents, and the officer reaching the office.
- Action 1: The assistant submitting the documents (happened first).
- Action 2: The officer reaching the office (happened second).

2. Examine tense usage:
- "The officer was unaware" (Part A) uses the simple past tense ("was"), which is correct.
- "that his assistant had already submitted the documents" (Parts B and C) correctly uses the past perfect tense ("had submitted") to indicate an action that was completed before another action in the past.
- "before he reach the office" (Part D) describes the subsequent past action. However, the verb "reach" is in the base form (present tense/infinitive form) instead of the simple past tense.

3. Correcting the error:
To maintain past tense consistency, the verb "reach" must be changed to the simple past tense form "reached".
Therefore, the corrected sentence should read: "The officer was unaware that his assistant had already submitted the documents before he reached the office."

Hence, the part containing the error is D.