Explanation:

Alright, here’s the thing—let’s go through each option and see what we can actually know.

- Option 1: the number of cheese sandwiches sold

Nope. All we know is the total number of sandwiches sold (21), but we don’t know how many of those were cheese, turkey, or egg salad.

- Option 2: the cost of a turkey sandwich

Definitely not. There's nothing about the price of any sandwich in the information given.

- Option 3: the total number of sandwiches that can be placed in the refrigerator

Again, not possible. We know how many are currently there (42), but nothing about the refrigerator’s capacity.

- Option 4: the number of sandwiches remaining in the refrigerator after the lunch period

Yes! Start with 42 sandwiches—21 are sold, so 21 are left. This is basic subtraction, and it’s directly supported by the facts.

Option 4 is the only thing you can actually determine. The rest? There just isn’t enough info. Simple as that.