Amartya Sen was awarded the Nobel Prize for his contribution to

The correct option is Welfare Economics.

Amartya Sen was awarded the Nobel Memorial Prize in Economic Sciences in 1998 for his outstanding contributions to welfare economics and social choice theory, as well as for his interest in the problems of society's poorest members.
Welfare economics is a branch of economics that uses microeconomic techniques to evaluate well-being at the aggregate level. Sen's work revolutionized the study of public choice, poverty indexes, and famines, establishing theoretical foundations for evaluating democratic decisions and individual welfare.