Answer
C .The final velocity is half of train car B’s initial velocity
Explanation
According to the law of conservation of momentum given by the product of mass and velocity, the velocity will be slightly that of the initial one due to increased mass (double mass). Here at least half of the kinetic energy is lost. Momentum is conserved but kinetic energy is converted to other energy forms after an inelastic collision. Kinetic energy is conserved in elastic collisions.