Choose the correct answer. Are they riding a bike?

The question uses the present continuous tense ('Are they riding'). Therefore, the correct answer should also use the present continuous tense. The options are: 'No, they doesn't.', 'Yes, they do.', 'Yes, they are.', 'Yes, they aren't.'. The grammatically correct response to 'Are they riding a bike?' is 'Yes, they are.' or 'No, they aren't.'. Among the given options, 'Yes, they are.' is the most direct and appropriate affirmative answer.