The best way to avoid dehydration among the options provided is to drink the amount of water lost in sweat. Staying properly hydrated by replenishing fluids lost through sweat is crucial, especially during physical activities or in hot weather.
While avoiding exercise in warm conditions and wearing appropriate clothing can help reduce the risk of dehydration, the most direct method to prevent dehydration is to consume enough fluids. Avoiding helmets does not effectively contribute to hydration, and wearing dark clothing can actually increase heat absorption, which is not beneficial in terms of managing temperature and hydration.