As my son . . . . . . . . down by the river, I . . . . . . . . his room thoroughly.

Solution (By Examveda Team)

In the sentence, "As my son was fishing down by the river, I cleaned his room thoroughly," the past continuous tense and the past simple tense are used to describe two actions happening simultaneously in the past.

The correct form for the sentence is "As my son was fishing down by the river, I cleaned his room thoroughly."

The other options are not suitable because they do not use the correct verb forms or tenses:

Option A: fishes/have cleaned
This option combines the present simple tense "fishes" with the present perfect tense "have cleaned," which is not appropriate for describing simultaneous past actions.

Option B: is fishing/cleaned
This option combines the present continuous tense "is fishing" with the past simple tense "cleaned," which does not accurately convey the simultaneous nature of the actions.

Option C: has fished/clean
This option combines the present perfect tense "has fished" with the base form "clean," which does not accurately represent the past continuous action.

Option D: fished/have been cleaning
This option combines the past simple tense "fished" with the present perfect continuous tense "have been cleaning," which is not suitable for describing simultaneous actions.

Therefore, the correct answer is Option E: was fishing/cleaned.