Step 1: Concept
For an incompressible flow, the divergence of the velocity field must be zero: $\nabla \cdot \overline{v} = 0$.
Step 2: Meaning
Calculate divergence: $\frac{\partial v_x}{\partial x} + \frac{\partial v_y}{\partial y} + \frac{\partial v_z}{\partial z} = a_1 + b_2 + c_3 = 0$.
Step 3: Analysis
Substitute the given values: $2 + b_2 + (-4) = 0$.
Step 4: Conclusion
Simplify: $b_2 - 2 = 0 \rightarrow b_2 = 2$.
Final Answer: (C)