What type of market utilizes code readers/diaries for ratings and is limited to 14 markets?

The correct answer pertains to a specific type of market research tool known as code readers/diaries. These are utilized to gather ratings in a structured manner for a limited number of markets, specifically 14 in this case. Code readers, also known as diary panels, require participants to record their consumption of media or products, which is then compiled and analyzed to provide insights into viewing habits or product usage.

This method is particularly effective for capturing detailed consumer behavior because participants document their actions in real-time, leading to more accurate and reliable data. The focus on just 14 markets suggests a specialized application, perhaps targeting areas that are significant for particular research purposes.

The other choices represent different methodologies or technologies used in market research but do not fit the specific criteria of using code readers/diaries and being limited to 14 markets. Therefore, option C is the most appropriate answer given the context of the question.