Why is the answer b for this problem?

B

Explanation:

This looks like a momentum question. It also looks like there is no horizontal acceleration.

Momentum before           Momentum after

m1vo                                   (m1 + m2)*vo/3           multiply through by 3

3 m1*vo                               m1* vo + m2*vo          subtract m1*vo from both sides

3m1*vo - m1*vo                   m2*vo

2m1*vo                                 m2*vo                      Divide by vo

2m1 =                                    m2

Conclusion: It takes 2 m1's to equal 1 m2.

B is the answer.