The field of political economy applies the methods of
a. economics to study how government works.
b. political science to study how the economy works
c. psychology to study how the economy works.
d. psychology to study how government works.

From the word itself, political economy is a branch of science that deals with the principles of economy like law of supply, law of demand, law of diminishing marginal utility and many more. It not only stops from there. They relate these economic principles to how it would conform to the laws of the government and the impact it will have on the national wealth and, eventually, the people.

Therefore, the answer is letter A.