Which type of agreement stipulates how a partner's stake is managed upon life events like death or retirement?

The correct choice is the buy-sell agreement because it specifically addresses the ownership transfer of a partner's interest in the partnership upon certain life events, such as death, retirement, or disability. This agreement outlines the process and terms under which a partner's stake can be bought out by other partners or the partnership itself, ensuring an orderly transition that helps maintain the stability and continuity of the business.

A partnership agreement generally outlines the relationship between the partners, including their duties, rights, and how profits are shared, but it may not specifically focus on the mechanisms for transferring ownership upon life events. An operating agreement, typically used in LLCs, describes the management of the entity and member duties but does not typically address life events related to partner ownership. An investment agreement primarily pertains to the relationship between investors and the entity with regards to investment terms, rather than the specific management of a partner's interest in the case of death or retirement.

Thus, the buy-sell agreement is the most precise tool for managing a partner's stake upon such significant life changes, ensuring all parties are clear on the process and valuation of the interest involved.