Let us walk through the options:
Option A: Systematic sampling
Systematic sampling is a type of probability sampling method in which sample members from a larger population are selected according to a random starting point but with a fixed, periodic interval.
This option is incorrect because the selection was not made with a fixed interval in mind
Option B: Convenience sampling
Convenience sampling is a type of nonprobability sampling in which people are sampled simply because they are "convenient" sources of data for researchers
This option is correct because the researcher selected the first 35 students leaving the gym because it's convenient
Answer: Option B