The correct answer is d. Portfolio manager.
An institutional investor is an organization or entity that invests significant amounts of money on behalf of others. They typically manage large pools of funds, such as pension funds, endowments, insurance companies, or mutual funds. A portfolio manager, who manages investment portfolios on behalf of institutional investors, falls under this category.
While investment bankers, security analysts, and stockbrokers are involved in the financial industry, they are not necessarily considered institutional investors themselves. Instead, they may work with or provide services to institutional investors.