You are part of the sample. Hence the correct option is a.
In the context of a survey from a 24-hour gym, the distinction between the sample and the population is essential. The population represents the entire group that the survey aims to draw conclusions about, which, in this case, could be all individuals who use or potentially could use the gym's services. As an individual who has filled in the survey, you are part of the sample, which is a subset of the population.
The sample consists of those individuals who have participated in the survey, providing insights and data that researchers can analyze to make inferences about the broader population. Therefore, by responding to the survey, you contribute to the sample, allowing the gym to gather information and draw conclusions about the health, exercise habits, and preferences of its clientele or target audience.