Explanation:

- "You must wear your helmet while riding a motorbike, but you may hurt seriously in case of an accident."

- "But" is used to contrast two ideas. Here, it doesn't connect logically with the need for safety.

- "You must wear your helmet while riding a motorbike, although you may hurt seriously in case of an accident."

- "Although" is used to show contrast, often to say something unexpected. It doesn't fit when explaining a precaution.

- "You must wear your helmet while riding a motorbike, even though you may hurt seriously in case of an accident."

- "Even though" is similar to "although" in meaning and use, and is also inappropriate here.

- "You must wear your helmet while riding a motorbike, otherwise you may hurt seriously in case of an accident."

- "Otherwise" is used to indicate a possible consequence. This makes sense as it shows the result of not wearing a helmet.

Correct Answer: Option 4 - otherwise