Step 1: Rule out the simple cases first. Rice and gram are food grains that reach the kitchen almost as they are, so the grower keeps most of the price. Milk usually moves through a short cooperative or vendor chain to the household.
Step 2: Cotton is not sold to the consumer in its raw form at all, it is only the starting material for cloth. Between the farm and the shop shelf sit ginners, spinners, weavers, processors and retailers, each taking a cut.
Step 3: With so many hands adding value and margin after the farm gate, the cotton grower's share of what the final buyer pays is the smallest of the four. $\text{Farmer's share} = \dfrac{\text{Price at farm gate}}{\text{Price paid by consumer}}$ is lowest for cotton. \[\boxed{\text{Cotton}}\]