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According to the law of conservationof momentum,
m1v1 + m2v2 = (m1+m2)V
2*4 + 4*1 = (2+4)V
V= 2 m/s
Now,
Total energy after collision = 1/2 (m1+m2) (V^2)
= 1/2 (6) (2^2)
= 1/2 (6) (4)
= 12 J
So, B is the answer.