The correct answer is C, hold the fund’s cash and securities. A mutual fund custodian is a financial institution, typically a bank, responsible for the safekeeping of the fund’s assets. This includes maintaining custody of the fund’s securities (such as stocks and bonds) as well as holding its cash. The custodian ensures that assets are secure, properly recorded, and protected from theft or misuse.

It is important to distinguish the custodian’s role from other service providers. The investment adviser or portfolio manager (choice A) is responsible for making investment decisions and managing the portfolio. The broker-dealer (choice B) executes trades by buying and selling securities on behalf of the fund. The transfer agent (choice D) handles recordkeeping functions related to shareholder accounts, such as processing purchases, redemptions, and maintaining ownership records.

The custodian plays a critical oversight and control function, helping ensure compliance with regulatory requirements and protecting investors’ assets. This separation of duties is a key safeguard in the mutual fund industry, reducing the risk of fraud or mismanagement.

Thus, the primary role of the custodian is safekeeping, making choice C the correct answer.