The correct answer is D. Knowledge about a subject
In education, achievement tests are a type of test commonly used to evaluate the skills and knowledge students have learned after a specific time, for example, a grade that can cover different subjects or focus on only one. Because of this, achievement tests are widely used in almost all educational levels as they show whether or not the student has learned in a class and how prepared the students for certain tasks that are determined according to the content of a subject. Considering this, it can be concluded achievement tests are designed to measure one's knowledge about a subject.
Answer: Carrying capacity
If a population overshoots its carrying capacity, it may have a population crash.
Population refers to the total number of people living or inhabiting in a particular district, city geographical area or country.
Carrying capacity refers to the number of people which an area's resources can sustain for an unlimited time or indefinitely without exhausting or degrading the resources.