In a within-subjects study that extends over a relatively long time, it is possible that there will be systematic changes in the participants’ skills or knowledge during the time of the study. When these changes influence the participants’ scores, causing scores at the end of the study to be different from scores at the beginning, the effect is called .A) a history effect.
B) instrumentation.
C) maturation.
D) regression toward the mean.

A) a history effect.          

Explanation:

History effect: In psychological research, the term "history effect" is described as different events that generally occurs in the specific environment which is responsible for changing the pre-existing conditions of a particular study, therefore affecting or hindering its outcome. However, these history events can occur either between post-test &pre-test or before starting a specific experiment.

In the question above, the given statement signifies a history effect.

    racial conflict and discrimination has existed ever since man learned to kill with rocks and bones. these conflicts arose from disputes between territory, power, wealth, religion, or simple difference. some of the most infamous racial disputes include slavery, the holocaust, and the civil rights movement.

    these listed disputes often resulted in casualties from all sides. riots, wars, imprisonment, and lynching are all commonly associated with racial conflict on severe levels.

      however, racism is an improving issue in recent decades. people are being more accepting and welcoming of diverse populations. though, the world isn't perfect and it will take much more time, if ever, until the world is rid of racism.