Apartnership agreement must be in writing if a. any partner contributes more than $500 in capital. b. the partners reside in different states. c. the partnership intends to own real estate. d. the partnership's purpose cannot be completed within one year of formation.

D. The partnership's purpose cannot be completed within one year of formation

Explanation:

Partnership: The partnership is an agreement that is made between two or more persons called partners. In this partnership agreement, the sharing of profit and losses should be distributed between their profit-loss sharing ratio

The agreement can be verbal or non-verbal. But to exist in the eyes of law, the agreement should be in writing.  

There are no conditions of the written agreement except when its motive is not completed within one year after coming in partnership. Because evidence of anything is more important than say in non-verbal. As anyone can stay back to protect themselves. So, to proof ourselves in the eyes of law, written agreement is much important.

So, there should always be written agreement instead of an oral agreement

All other reasons which are given in the question are invalid. Hence, ignored it