Over twenty years ago, in a landmark experiment in the psychology of choice, professor Sheena Iyengar

Choice A is the best answer. The convention being tested is the use of a colon within a sentence. In this choice, the colon is used in a conventional way to introduce the following description of how the number of jams available varied. 

Choice B is incorrect because it creates a comma splice. A comma can’t be used in this way to join two main clauses (“the number…varied” and “some…six”). Choice C is incorrect because it results in an illogical and confusing sentence. Using the conjunction “while” to join the main clause (“the number…varied”) with the following clause’s description of the number of jams available suggests that the variation in the number of jams is in contrast to some shoppers having twenty-four options. Choice D is incorrect because it results in an illogical and confusing sentence. Using “while” in this way suggests that the number of jams available varied during the time in which some shoppers had twenty-four options and others had six. The sentence makes clear, however, that what follows “varied” is a description of the variation, not a separate, simultaneous occurrence.