The correct answer is B) increase the rate of heat loss from the body. Physical exertion in cold water leads to increased blood flow to the body's extremities, which in turn increases the rate of heat loss from the body's core. This accelerates the onset of hypothermia, which can rapidly become life-threatening. Remaining still and conserving body heat is the best strategy for survival in cold water, rather than increasing physical activity. Options A, C, and D are incorrect because they do not accurately describe the physiological effects of cold water exposure and physical exertion.