Problems that require a common object to be used in an unusual way may be difficult to solve because of: A. mental set. B. irrelevant information. C. unnecessary constraints. D. functional fixedness.

To choose the correct answer, let's first define each concept: A. Mental Set: A mental set is a habitual and rigid way of solving a problem that limits our ability to think about new and alternative solutions. B. Irrelevant Information: Irrelevant information is the information that is not important or relevant to the current problem and can distract or mislead a person from finding the correct solution. C. Unnecessary Constraints: Unnecessary constraints refer to limiting factors or assumptions that are self-imposed by the problem solver and can make it harder to solve the problem. D. Functional Fixedness: Functional fixedness is a cognitive bias that hinders one's ability to see an object being used for something beyond its typical use. It limits our creativity in finding alternative solutions to problems.

The question refers to problems that require a common object to be used in an unusual way. This means we are looking for a concept that is associated with the difficulty of using objects beyond their typical function.

Among the options provided, the concept that best matches the question is functional fixedness. This cognitive bias makes it difficult for someone to see an object being used for something other than its intended purpose. Therefore, the correct answer is: D. Functional Fixedness.