What are the possible combinations that can be made

For the president, you have 3 possible choices: A, B, or C. Once you’ve made that first choice, you can choose between the two remaining options for the Vice President (for example, if A was President, you could choose between B and C). Once you’ve made that second choice, there’s only one left to give the role of treasurer (if A is president and B is Vice President, C has to be treasurer).

Altogether, you have 3 • 2 • 1 = 6 ways you can assign roles, and all 6 are represented in the last multiple-choice answer.