Tim . . . . . . . . a master's degree as soon as he . . . . . . . . to Canada.

In the sentence, "Tim began a master's degree as soon as he returned to Canada," the past simple tense "began" is used to describe an action that happened in the past (starting a master's degree), and the past simple tense "returned" is used to describe another action that also happened in the past (coming back to Canada). The context suggests that both actions occurred sequentially.

The correct form for the sentence is "Tim began a master's degree as soon as he returned to Canada."

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

Option A: has begun/is returning
This option combines the present perfect tense "has begun" with the present continuous tense "is returning," which does not accurately convey the sequence of events in the past.

Option C: is beginning/returned
This option combines the present continuous tense "is beginning" with the past simple tense "returned," which is not appropriate for describing a sequence of actions in the past.

Option D: was beginning/returns
This option combines the past continuous tense "was beginning" with the present simple tense "returns," which is not suitable for describing past actions and their sequence.

Option E: begins/has returned
This option combines the present simple tense "begins" with the present perfect tense "has returned," which does not accurately convey the sequence of past actions.

Therefore, the correct answer is Option B: began/returned.