What factor may threaten a study's internal validity due to changes occurring outside the intervention?

The correct answer is history, as it refers to external events or changes that occur during a study that can affect the participants or the outcome of the intervention. These events are not related to the intervention itself but can influence the results. For example, if a social work study assessing the effectiveness of a new counseling technique spans several months, significant social or political changes during that time—such as a natural disaster or a major policy shift—could impact the participants' experiences and responses, thus confounding the results.

Other factors mentioned such as maturation pertains to changes within the subjects themselves over time, such as natural developmental changes, which can also affect results. Testing effects refer to the potential influence of taking a test multiple times on participants' scores, potentially skewing results. Experimental mortality, or attrition, involves participants dropping out of the study, which can alter the population being studied and affect the validity of the findings. However, it is the concept of history that best captures the idea of outside influences threatening internal validity in a study's context.