The part of experiment that the researcher is manipulated is called:

Correct option is C

The part of an experiment that the researcher manipulates is called the Treatment Variable (also known as the Independent Variable). This is the variable that is changed or controlled to test its effects on the dependent variable. The researcher deliberately manipulates this variable to observe how it influences the outcome of the experiment.
The other options are explained as follows:
Dependent Variable (a): This is the outcome or response that is measured in the experiment. It is what the researcher is trying to explain or predict and is affected by changes in the treatment variable.
Third Variable (b): This is an extraneous variable that is not the focus of the experiment but may have an indirect effect on the dependent variable. It can create a spurious relationship between the independent and dependent variables.
Extraneous Variable (d): These are variables that are not of primary interest in the experiment but could affect the dependent variable. Researchers try to control extraneous variables to isolate the effect of the treatment variable on the dependent variable.
Information Booster:
The Treatment Variable is also known as the Independent Variable because it is the one manipulated to observe its impact on the dependent variable.
The Dependent Variable depends on the variations in the treatment variable and is the main focus of the study.
Extraneous Variables should be controlled to avoid them confounding the relationship between the independent and dependent variables.
A Third Variable can complicate the interpretation of an experiment if it is not controlled, as it can cause a false appearance of a relationship between the independent and dependent variables.