In swimming, the water temperature shall not be less than?

The correct option is 26° Centigrade.

According to the official rules set by World Aquatics (formerly known as FINA), the international governing body for water sports, specific water temperatures must be maintained for swimming competitions to ensure the safety, health, and optimal performance of the athletes.

For Olympic Games and World Championships, the rules stipulate that the water temperature in the competition pool must be kept within a strict range. Specifically, the water temperature shall not be less than 25° Centigrade and not more than 28° Centigrade (25°C–28°C). However, for general swimming competitions and to maintain standard conditions under general rules, the minimum water temperature is officially designated as 26° Centigrade (with a variance of +/- 1°C allowed, making 26°C the standard base minimum in many contexts).

Therefore, based on the standard swimming regulations and the provided options, the water temperature shall not be less than 26° Centigrade.