The correct answer is D. Experimental research.
In experimental research, the researcher manipulates one or more variables to see how they affect another variable. This is done in order to test a hypothesis, which is a statement about the relationship between two or more variables.
Historical research is the study of past events. It does not involve manipulation, as the researcher cannot change the past.
Fundamental research is also known as basic research. It is the type of research that is conducted to gain knowledge about a particular topic, without any specific application in mind. It does not involve manipulation, as the researcher is not trying to test a hypothesis or to see how one variable affects another.
Descriptive research is the type of research that is conducted to describe a particular group of people or situation. It does not involve manipulation, as the researcher is not trying to change anything or to test a hypothesis.
In conclusion, the only type of research that always involves manipulation is experimental research.