Final answer:
The correct answer to fill in the blanks is (b) 'lost; had bought' because the past perfect tense 'had bought' signifies a completed past action before another took place, which is 'lost' - a simple past tense.
Step-by-step explanation:
The correct answer to fill in the blanks in the sentence, "He __ his mobile phone, shortly after he _____ it." is option (b) 'lost; had bought'. In this context, 'had bought' is used because the action of buying the phone happened first, followed by the action of losing the phone. The Past Perfect tense 'had bought' indicates a completed past action before another took place. The Simple Past tense 'lost' indicates the event happened after buying the mobile phone.
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