Physical or mental changes that may occur within the subjects over a period of time is called

The correct answer is Maturation. This term describes the internal physical or mental changes that naturally occur within subjects over time, independent of any experimental intervention. These changes, such as growing older, becoming more experienced, or simply getting tired, can influence the study's outcomes and pose a threat to internal validity if not accounted for. In contrast, Internal Validity refers to the study's ability to establish a causal link, while External Validity concerns generalizability. History refers to specific external events affecting participants during the study, not internal changes.