A sample is a set of individuals selected from a statistical population by a defined proceedure.
A group of 300 adults with specific characteristic was selected (the statistical population is then adults between 25 and 64 years).
The experiment involved that group of 300 adults, not the research team.
Furthermore, if one wants to do any kind of statistic and obtain a result from that, one need to consider all the elements in the sample as well as the outlayers (people that received 300mg of caffeine and yet can recall the images well enough) if any.
So, the answer is the 300 adults, ages between 25 and 64, who participated in the study