Ans. (B). conserve water.
Cactus plants grow in dry and arid conditions. These plants have adapted to perform CAM (crassulacean acid metabolism) photosynthesis, which helps them to conserve water.
Plants that use CAM pathway open their stomata during night to take CO2 for photosynthesis, while their stomata remain closed during the day to reduces evaporation or transpiration and conserve water.
Thus, the correct answer is option (B).