Here’s why:

Replication: Repeating an experiment with different samples or under slightly different conditions to verify the results and increase confidence in the findings. This is exactly what the researcher is doing by conducting three experiments with 100 subjects each instead of one large experiment.
Manipulation: Involves changing independent variables to observe their effect on dependent variables. While the researcher might be manipulating variables in their experiments, “replication” focuses on the specific act of repeating the entire experiment for validation.
Observation: Simply collecting data through watching or recording behavior without manipulating any variables. The researcher is likely going beyond passive observation by manipulating variables and collecting data in their experiments.
Validation: The process of confirming the accuracy and generalizability of findings. Replication is a key part of the validation process, but it’s not the entire process itself.
Therefore, replication best captures the concept of repeating the experiment with smaller groups to strengthen the overall research findings.