The correct answer is $\boxed{\text{B. }9.6\text{ t}}$.
The reaction at the support B is the force that the support exerts on the beam to prevent it from rotating. The magnitude of the reaction force can be calculated using the following equation:
$$R_B = \frac{wL}{2} + V$$
where $w$ is the load per unit length, $L$ is the length of the beam, and $V$ is the vertical force at the end of the beam.
In this case, $w = 2\text{ t}/\text{m}$, $L = 4\text{ m}$, and $V = 8\text{ t}$. Substituting these values into the equation, we get:
$$R_B = \frac{(2\text{ t}/\text{m})(4\text{ m})}{2} + 8\text{ t} = 9.6\text{ t}$$
Therefore, the reaction at the support B is $\boxed{\text{B. }9.6\text{ t}}$.
Option A is incorrect because it is the magnitude of the reaction force at the support A. Option C is incorrect because it is the magnitude of the vertical force at the end of the beam. Option D is incorrect because it is the magnitude of the load per unit length.