Answer:

To determine the correct answer to the question, "______ they swimming when the phone rang?", we need to consider the tense of the verb and the context of the sentence.

Tense:
The verb "swimming" is in the present continuous tense, which is formed using the helping verb "be" in the past tense (were) followed by the present participle form of the main verb (swimming). This tense is used to describe an ongoing action that is happening at the time of speaking or a specific point in the past.

Context:
The phrase "when the phone rang" provides the context for the action of swimming. It suggests that the action of swimming was interrupted by the ringing phone. In this case, we are talking about a specific point in the past when the phone rang.

Options:
a) Were
b) Are
c) Do
d) Did

Explanation:
Among the given options, the correct answer is option 'a' - "Were." This is because the verb "be" needs to be in the past tense to match the context of the sentence. The correct form of the sentence would be "Were they swimming when the phone rang?" which indicates that the swimming action was ongoing at the time the phone rang, and it was interrupted by the phone call.

Option 'b' - "Are" is incorrect because it is in the present tense and does not match the context of the sentence, which is referring to a specific point in the past.

Option 'c' - "Do" is incorrect because it is in the present tense and does not match the context of the sentence, which is referring to a specific point in the past.

Option 'd' - "Did" is incorrect because it is in the past tense, but it is used for forming questions in the simple past tense, not the present continuous tense. The sentence would be grammatically incorrect if "did" is used in this context.

Therefore, the correct answer is option 'a' - "Were."