Choose the correct option. While my dad __ in the garage, my brothers __ tennis.

The sentence describes two simultaneous actions in the past. The first action, 'my dad __ in the garage,' is a continuous action that was in progress. The second action, 'my brothers __ tennis,' is also a continuous action that was happening at the same time. Therefore, the past continuous tense is appropriate for both. The correct option is 'was working / were playing'.