Final answer:
The question asks to find the number of combinations possible when selecting 4 tax returns out of 9 for an audit, which can be calculated using the combinations formula. The correct answer is 126 ways.
Step-by-step explanation:
The question is regarding the number of ways an IRS auditor can select 4 of 9 tax returns for an audit. This is a problem of combinatorics, which is a field within mathematics that deals with counting combinations and permutations. The precise topic here is combinations since the order of selection does not matter.
To calculate the number of ways to choose 4 returns out of 9, we utilize the combination formula which is C(n, k) = n! / (k! * (n - k)!), where 'n' is the total number of items, 'k' is the number of items to choose, and '!' denotes factorial.
For this case, C(9, 4) = 9! / (4! * (9 - 4)!) = 9! / (4! * 5!) = (9 * 8 * 7 * 6) / (4 * 3 * 2 * 1) = 126. Therefore, there are 126 ways the auditor can select 4 tax returns for an audit, which corresponds to option a.