Step 1: Identify the tense clue. The verb 'called' shows this happened in the past. Step 2: Identify the relationship between the two actions. One action (doing homework) was ongoing when another action (mother calling) interrupted it. Step 3: Apply the rule. An ongoing past action interrupted by another past action uses the past continuous tense: subject + was/were + verb-ing. Step 4: Form the verb. Siti (singular) + 'was doing'. Answer: was doing