The correct answer is - C. light.
The cloudy waters are waters where the visibility is very low. That is a result of higher amount of sediments or organic debris in the water. Because the water has low visibility, it means that the light can not reach very deep into it, only few centimeters on the surface of it. The plants, including the sea grass, are using the light for the process of photosynthesis which is crucial process for their survival as through it they are producing their own food, or rather the sugars. With the light being insufficient or lacking, the sea grass will die out in those parts, and will only be able to exist in the parts where there's enough light.
