5типов вопросов к каждому предложению:
1) he is washing the dishes now.
2. they often visit the british museum.

1) He is washing the dishes now.

Is he washing the dishes now? (Общий вопрос)

Is he washing the dishes or watching TV now (Альтернативный вопрос)

What is he washing now? (Специальный вопрос)

Who is washing the dishes now? (Вопрос к подлежащему)

He is washing the dishes now, isn't he? (Разделительный вопрос)

2) They often visit the British museum.

Do they often visit the British museum? (Общий вопрос)

Do they often visit the British museum or go to the central park? (Альтернативный вопрос)

What do they often visit? (Специальный вопрос)

Who often visits the British museum?  (Вопрос к подлежащему)

They often visit the British museum, don't they? (Разделительный вопрос)