The correct answer is option 'D', which states that a partnership at will can be dissolved only on the happening of an event. Let's discuss each option and understand why option 'D' is the correct answer.

1. Duration not fixed: This option is not correct because a partnership at will does not have a fixed duration. It means that there is no predetermined time period for the partnership to exist. It can continue as long as the partners wish to work together or until it is dissolved.

2. Duration fixed: This option is not correct because a partnership at will does not have a fixed duration. As mentioned earlier, it can continue for as long as the partners want to work together.

3. Dissolved at any time: This option is partially correct. A partnership at will can be dissolved at any time by any partner without giving any prior notice or reason. However, it is not the only way a partnership at will can be dissolved.

4. Can be dissolved only on the happening of an event: This option is the correct answer. A partnership at will can be dissolved when a specific event occurs, such as the death or withdrawal of a partner. In this case, the partnership will be automatically dissolved. However, it is important to note that the partnership can also be dissolved by any partner at any time, as mentioned in option 3. So, while option 3 is true, option 4 provides a more comprehensive answer by including both the event-based dissolution and the ability of any partner to dissolve the partnership.

In conclusion, a partnership at will is a type of partnership where the duration is not fixed, and it can be dissolved at any time by any partner or on the happening of an event. Therefore, option 'D' is the correct answer as it includes both the event-based dissolution and the partner's ability to dissolve the partnership.