Which of the following is most likely to be a fixed cost?

The only cost on here likely to be a fixed cost is how much you pay in rent. This is usually fixed from month to month, and is among the first things to come out of a paycheck or out of the profits made from a business.

Cost of materials, commissions, and assembly labor costs are going to change depending on the nature of what is going on from one month to the next.

That said, these other answers are going to be in the same ballpark from month to month. Commissions might be the only thing that wouldn’t be that easily predictable, depending on what business you’re in. That’s just the way the business world runs.