6. While I studied yesterday morning, Tom dropped by to see me. Choices: were studying / was studying; was dropping / were dropping.

Перевод задания: Простое прошедшее или прошедшее длительное время.

While I was studying yesterday morning, Tom dropped by to see me.
Пока я занимался(ась) вчера утром, Том заглянул ко мне.

После while указано длительное действие — was studying; короткое событие — dropped by — ставится в Past Simple.