Answer :
To find out whether breast milk feeding contributes to the intellectual development of children, psychologists would most likely use option d, experiments.
Experimental research manipulates one or more variables to determine their effect on the other variable. It is used to establish a cause-and-effect relationship between the variables. Experimental research is conducted in a controlled environment and uncontrolled environment to investigate the hypothesis closely.
Lab experiments are an example of a controlled environment to conduct experimental research. Field experiments and natural experiments are conducted in an uncontrolled environment with less control over the variables. The psychologist will experiment on the effect of breast-milk feeding on the intellectual development of children by manipulating the breast-milk feeding. Here, intellectual development will be an independent variable.
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