Final answer:
Solving investment problems is best represented by an integer problem because they can involve gains or losses represented as positive or negative integers.
Step-by-step explanation:
The scenario that is best represented by an integer problem is C. Solving some investment problems. This is because investments often involve gains or losses of money, which can be represented as positive or negative integers. In contrast, scheduling workers (option A) is not typically represented by integers, buying large items such as planes (option B) often involves very large sums and decimals representing cents, and manufacturing of items (option D) usually requires more detailed continuous data rather than integers.