The correct answer is D. Water-cement ratio.
The water-cement ratio is the ratio of the weight of water to the weight of cement in a concrete mix. It is one of the most important factors affecting the strength of concrete. A higher water-cement ratio will result in a weaker concrete, while a lower water-cement ratio will result in a stronger concrete.
The quality of water does not have a significant effect on the strength of concrete, as long as the water is clean and free of harmful chemicals. The quantity of aggregate also does not have a significant effect on the strength of concrete, as long as the aggregate is clean and free of harmful materials. The quantity of cement does have an effect on the strength of concrete, but it is not as significant as the water-cement ratio.
The water-cement ratio is the most important factor affecting the strength of concrete because it controls the amount of hydration that occurs. Hydration is the chemical reaction between cement and water that produces the strength of concrete. The more water that is available for hydration, the stronger the concrete will be. However, too much water can weaken the concrete by making it more porous.
The ideal water-cement ratio for a given concrete mix will vary depending on the type of cement, the type of aggregate, and the desired strength of the concrete. The water-cement ratio should be determined by a qualified concrete engineer.