Final answer:
The threat to internal validity that can emerge when external events co-occurring with the independent variable affect outcomes is called history. History refers to specific events or occurrences that happen during the course of a study, which can influence the outcome. the correct option is a, History.
Step-by-step explanation:
The threat to internal validity that can emerge when external events co-occurring with the independent variable affect outcomes is called history. History refers to specific events or occurrences that happen during the course of a study, which can influence the outcome. These events are unrelated to the independent variable but can still impact the results, making it difficult to determine whether the effects observed are solely due to the independent variable.
For example, let's say you are conducting a study on the effectiveness of a new teaching method on student performance. This external event could have an impact on the outcomes of your study and make it difficult to attribute any changes solely to the new teaching method.