Answer: Regression
Explanation: Regression is the activity in which something returns to the state of low development.The measure of regression is seen when the stage of something reaches the previous level as compared to present.
The situation mentioned in the question also displays regression in the study because students have returned to the low stage regarding to marks in final exams as compared with mid-term exams.
Other options are incorrect because attrition is reduction in effectiveness of something, maturation is having matured state and observer bias is researching in bias manner. Placebo effect is the effect due to inert treatment with produces unknown consequences.