Final answer:
The reactive effect of testing that threatens external validity is most closely related to repeated measurement, a factor that can affect the participants' responses due to their awareness of being tested multiple times.
Step-by-step explanation:
The reactive effects of testing in threats to external validity include a range of factors that can influence outcomes. These are specifically related to how participants react to the fact that they are being studied. In the options provided, 3) Repeated measurement is the one that corresponds most closely to the challenges posed by testing in experimental design. Such reactivity can present itself through demand characteristics, where subjects act differently because they know they are being watched, or through purposeful distortion, where participants purposefully change their behavior. Repeated measurement pertains to the fact that when participants are tested multiple times, they may become sensitized to the testing process, which can affect their responses and thereby pose a threat to the study's validity. Irregularities in the object being measured and other factors that affect the outcome are also important aspects of scientific inquiry but pertain more to the accuracy of the measurements than to the reactive effects specifically caused by the testing process itself.