Question

the variable a scientist manipulates in an experiment is called the
independent variable
dependent variable
observation variable
controlled variable

Explanation:

In experimental design (a concept in Natural Science, especially in scientific method - related subfields like Biology, Physics, etc.), the independent variable is the one that the experimenter manipulates to observe changes in the dependent variable. The dependent variable is what is measured/observed, observation variable is not a standard term for this context, and controlled variables are kept constant. So the variable a scientist manipulates is the independent variable.

Answer:

A. independent variable