Step 1: Let: $S$ represent the weight of Sohan. $M$ represent the weight of Mohan. $R$ represent the weight of Rohan.

Step 2:

From S1: $2S<M$ or $2S<R$ This implies that Sohan's weight is less than half of either Mohan's or Rohan's weight.

From S2: $2R>M$ or $2R>S$ This implies that Rohan's weight is more than half of either Mohan's or Sohan's weight.

Step 3:

From S1: If $2S<M$, then $S<\frac{M}{2}$. If $2S<R$, then $S<\frac{R}{2}$. Therefore, Sohan's weight is less than both Mohan's and Rohan's weights.

From S2:

If $2R>M$, then $R>\frac{M}{2}$. If $2R>S$, then $R>\frac{S}{2}$. Therefore, Rohan's weight is greater than both Mohan's and Sohan's weights.

Step 4:

From the analysis, we conclude that Sohan's weight is the least among the three. However, we cannot definitively determine whether Mohan or Rohan has the greatest weight based on the given information.

The correct answer is D. 'Whose weight is greatest' cannot be determined.