Prof. Markowitz developed the analytical framework of the concept of "efficient frontier." The framework starts with the idea of an investment opportunity set, which is defined as:

a$.$ The collection of possible risk$-$return combinations available from a portfolio of individual assets.

b$.$ The collection of possible risk$-$return combinations available from a full diversified portfolio of individual assets.

c$.$ The collection of possible risk$-$return combinations available from a portfolio of individual assets that must include both stocks and bonds.

d$.$ None of the above.