To solve this problem, we need to understand the relationship between electric potential and electric field.$\\$Given:$\\$• The electric potential $V$ is constant at $5$ V throughout the region.$\\$• The volume of the region is $0.2$ m$^3. \\$The electric field $E$ is related to the electric potential $V$ by the equation:$\\ E = -\nabla V \\$Since the electric potential is constant throughout the region, its gradient is zero.$\\ \nabla V = 0 \\$Therefore, the electric field $E$ is:$\\ E = -0 = 0 \\$Thus, the magnitude of the electric field in this region is $0$ N/C.$\\$This corresponds to Option 4.