D

Step 1: Analyze the expressions for E1 and E2.
E1 = x \frac{dV}{dx} + y \frac{dV}{dy}:
This expression contains variables x and y, which suggests that it varies with x and y and thus does not represent a constant electric field because the electric field is constant only if the potential gradient is uniform.

E2 = xy^2 \frac{dV}{dx} + x^2y \frac{dV}{dy}:
Similarly, this expression also involves products of variables x and y, which makes it variable dependent and indicates it does not form a constant electric field across the region.

Step 2: Conclusion:
Therefore, both E1 and E2 do not represent a constant electric field. The correct answer is D: none of these.