Answer:
Combination, but keep in mind that if the committee had two open positions, say President and Secretary, it would be a permutation
Step-by-step explanation:
The first thing to keep in mind is the difference between combination and permutation.
The main difference is that in the combinations the order does not matter, whereas in the permutations the order does matter.
Combination example:
Choose 7 people for a project.
Example of permutation:
Choose 5 men for each specific role in a soccer team.
Therefore, "group of 5 senators is chosen to be part of a special committee" is a combination, but keep in mind that if the committee had two open positions, say President and Secretary, it would be a permutation.
