If Harry has a probability of being chosen as treasurer, what sequence must occur for him to become secretary instead?

To understand why the correct choice is that Harry must not be chosen as president before becoming secretary, consider the roles and the sequence of selection involved in this scenario. If there is a hierarchy or a predefined order in which positions are filled (for example, president, followed by secretary), then being selected for a prior role would prevent him from being elected to a subsequent role.

If Harry were to be chosen as president first, it would typically mean he cannot concurrently fill the role of secretary, effectively blocking him from that position. The concept here hinges on the structure of how the roles are filled and the assumption that one person cannot serve in multiple positions at the same time.

In this context, the other options don't address the necessary condition that Harry needs to avoid the role of president to open up the possibility of him being elected as secretary. Thus, the appropriate condition that must occur for Harry to be able to pursue the secretary position instead is indeed ensuring he is not selected as president beforehand.