Ответ: соглашусь с предыдущим комментарием. Тут должен быть либо полностью Present Simple (they buy bread every day), либо полностью Present Continuous (they are buying bread now(к примеру)).

Объяснение:

1. General:

Do they buy bread every day? Yes, they do.

2. Special:

Who buys bread every day? They do.

3. Alternative:

Do they buy bread every day or every week? They do every day.

4. Disjunctive:

They buy bread every day, don't they? Yes, they do.