Institution: বরিশাল বিশ্ববিদ্যালয় Exam: খ ইউনিট || বরিশাল বিশ্ববিদ্যালয় || 2014
Topic: Fill in the Blank, Subject: English Grammar (ইংরেজি গ্রামার)

Best Answer Explained in Below-:

The question "What.......... at ten o'clock when I phoned?" requires a specific grammatical structure that reflects an ongoing action in the past. When we analyze the sentence options, we see that "were you doing" is the only choice that correctly uses the past continuous tense.

1. **Past Continuous Tense**: This tense is formed using "was/were" plus the verb ending in -ing. It describes actions that were happening at a specific moment in the past. In this case, "at ten o'clock when I phoned" indicates a point in time during which an action was ongoing, making "were you doing" appropriate.

2. **Subject-Verb Agreement**: The structure "were you" is in the correct interrogative format for the second person. This aligns with the informative nature of the question, which is asking for details about what someone was engaged in at a precise moment.

3. **Other Options**:
- A) "did you do": This structure indicates a completed action, which does not fit with the timing of "at ten o'clock" when an action was ongoing.
- B) "you were doing": While it is correct in terms of grammar, it lacks the interrogative form required to ask a question.
- D) "did you": This is a simple past structure and does not correspond to the continuous action implied by the context of the question.

Thus, "C) were you doing" is the only grammatically correct option that conveys the intended meaning of ongoing action during a specific time in the past.