Portfolio immunization is a strategy that is designed to protect the returns on purchased securities, regardless of the direction in which interest rates move. This strategy is primarily concerned with balancing the risks associated with price and reinvestment in an investment portfolio. The goal of portfolio immunization is to ensure that the portfolio is not adversely affected by changes in interest rates, thereby safeguarding the returns on the securities purchased. This is achieved by carefully selecting securities that have offsetting price and reinvestment risks. In other words, if the price of a security falls due to an increase in interest rates, the loss is offset by the higher reinvestment return, and vice versa. This ensures that the total return on the portfolio remains stable, irrespective of changes in interest rates.

Choice A is incorrect. Portfolio shifting involves changing the composition of a portfolio in response to changes in market conditions or investment goals. It does not necessarily aim to safeguard returns on purchased securities against interest rate fluctuations.

Choice B is incorrect. Duration is a measure of the sensitivity of the price of a bond or other debt instrument to a change in interest rates. While it can be used as part of an immunization strategy, duration itself does not describe a strategy that aims to safeguard returns on purchased securities irrespective of interest rate fluctuations.