The reaction at the central support B of the beam ABC hinged at D shown in below figure is A. 2 t B. 5.8 t C. 0.2 t D. 3.5 t

The correct answer is $\boxed{\text{B. }5.8\text{ t}}$.

The reaction at the central support B of the beam ABC hinged at D is the force that the support exerts on the beam to keep it from rotating. The magnitude of the reaction force can be calculated using the following equation:

$$R_B = \frac{wL}{2}$$

where $w$ is the weight of the beam per unit length, $L$ is the length of the beam, and $R_B$ is the reaction force at B.

In this case, $w = 2\text{ t}/\text{m}$, $L = 10\text{ m}$, and therefore $R_B = 5.8\text{ t}$.

Option A is incorrect because it is the weight of the beam. The reaction force at B is not equal to the weight of the beam.

Option C is incorrect because it is the reaction force at D. The reaction force at D is not equal to the reaction force at B.

Option D is incorrect because it is the reaction force at C. The reaction force at C is not equal to the reaction force at B.