Dave's marketing research returned the finding that customers were staying away from his bookstore because of a lack of services like gift cards, return policies, and special orders. Dave was shocked. "Nobody ever asks about that stuff! If it were that important, people would ask about it." Dave is likely suffering from a(n) gap.

Knowledge gap

Explanation:

 The knowledge gap is one of the concept that helps in explaining the lack of knowledge about the specific concept and by identifying our own abilities, skills and knowledge we can easily identity our main factor of the lack of knowledge.

According to the given question, Dave is basically suffering from the knowledge gap as Dave is unaware about the fact that why people are satisfying away from his store.

 Based on the marketing research method he analyze that due to the lack of various types of services such as no return policies, gift cards offers and also the various types of special discount offers the people shows no interest in his book store.

 Therefore, Knowledge gap is the correct answer.