Your question asks what Incidence rate is.
The reason why answer choice "D). percentage of the general population that is the subject of the market research" would be the correct answer because this is the best answer to the description of an incidence rate.
An incidence rate is the amount of people that is "eligible" to participate in market study, or research.
Researches get the incidence rate of the general population in order to see how much people could participate in the research; showing how much people actual does and/or own the thing that they're researching about.
For example, a group of researchers could be researching about Toyota cars and how well they drive on a daily basis. In order to be eligible for the research on Toyota, you must own a Toyota. This means that the incidence rate is the amount of people that own a Toyota.
