Answer: Maturation
Step-by-step explanation:
In terms of internal validity, maturation is type of treat that impacts research because certain physical changes or mental modification tend to occur in participants which tends to alter the results.
According to the question, Stanley should worry about maturation trait as he is conducting experiment on first grade student .He conducts research of teasing on them in first week and then three months later which could have brought some mental changes or physical changes in student .Thus results can get modified.