Concept:Water holding capacity of a soil determines how much water it can retain, and this retained water adds to the weight of the soil sample.
Explanation:When equal amounts of water are poured on dry soil samples, the sample that absorbs and holds the maximum water will show the highest increase in weight per unit amount of soil.
Clayey soil has very fine particles and large surface area, so it can hold more water than sandy, loamy, or gravelly soil.
Gravel has the lowest water holding capacity, while sandy and loamy soils retain less water than clayey soil.
Therefore, the increase in weight per unit amount of soil is highest for clayey soil.
Answer:B. Clayey soil.