Which type of fund's shares can be purchased and sold only at its NAV without restriction?

Open-end funds are investment funds that allow investors to buy and sell shares directly from the fund itself at the net asset value (NAV) at the end of each trading day. This is a key feature of open-end funds. When investors wish to purchase shares, they do so at the current NAV, and similarly, when redeeming shares, they receive the NAV at the time of the transaction, which is calculated at the end of the trading day.

This structure contrasts with closed-end funds, which have a fixed number of shares and trade on exchanges like stocks, leading to market prices that can be above or below NAV. Index funds are a type of open-end fund that follows a specific index, but they still share the same characteristic of being purchased and sold at NAV. Hedge funds, on the other hand, often impose additional restrictions and are typically available only to accredited investors, further separating them from the open-end structure. Therefore, the defining feature of open-end funds makes them the correct answer to the question regarding purchasing and selling shares strictly at NAV without restrictions.