The correct answer is 30
In this experiment in question a total of 30 participants will be used. An experiment is characterized by observing or doing something under a certain "condition", which will result in a final outcome or state of events that are not predictable.
The experiments (commonly carried out in the field of science) are not precisely repeatable, even if carried out under supposedly identical and previously established conditions.
The experiments involve eventuality. And as they cannot be predicted (because they are random), they result in a set of results called "sample space" where the knowledge of the probability will be involved, which will indicate the frequency - higher or lower - in which the set of results occurs.