In Dunker's candle experiment, subjects weren't able to find a solution to the candle problem because they could not see past the conventional use of the box. Which obstacle to problem solving did the subjects experience?

Functional fixation

Explanation:

Functional fixation: In psychology, the term functional fixation refers to the cognitive function or cognitive bias in which a person becomes fixated on a particular function related to an object that prevents him or her from utilizing that specific object in any other way for problem-solving because the person feels that the object works only in a traditional way.

Example: A person uses an object in only a specific way and finds difficulty in using it in an alternative way.

In the question above, the subject experiences functional fixation.