The fund share of a closed-end fund is fixed and may not be increased or decreased without the approval of legal procedures during the closed-end period; Because the scale of open-end funds is not fixed, investors can apply for subscription or redemption at any time, and the fund share will increase or decrease accordingly. Option c is correct. Open-end fund refers to a fund operation mode that the fund share is not fixed and can be purchased or redeemed at the time and place agreed in the fund contract. Option a is wrong.

The buying and selling price of open-end funds is based on the net value of fund shares and is not affected by the relationship between market supply and demand. Option b is wrong. Closed-end funds generally have a fixed duration, while open-end funds are generally uncertain. Option d is wrong.