A lock-up period in hedge funds is a predetermined time span during which investors are not allowed to redeem or sell shares. The lock-up period is typically set at the inception of the fund and can range from a few months to a few years. This period is implemented to give the hedge fund manager the freedom to implement their investment strategy without the concern of withdrawals. It also protects the fund and its investors from the potential negative effects of large, unexpected redemptions. During the lock-up period, the fund manager can invest in less liquid assets, which might have higher returns, without worrying about liquidity for redemptions. After the lock-up period ends, investors can typically redeem their shares at regular intervals, such as quarterly or annually.

Choice A is incorrect. Redemption restriction is a broad term that refers to any limitation on the investor's ability to withdraw their funds from an investment. While it could technically include a lock-up period, it also includes other types of restrictions such as notice periods and redemption fees. Therefore, it does not specifically describe the restriction mentioned in the question.

Choice C is incorrect. Non-withdrawal period is not a term commonly used in the hedge fund industry to describe a specific time during which investors are not allowed to withdraw their funds.

Choice D is incorrect. High-water mark refers to a provision in hedge fund agreements that protects investors from paying performance fees on reinvested money that has previously been lost and recovered. It does not refer to any kind of withdrawal restriction.