Topic: Collisions
Concepts: Center of mass, Conservation of linear momentum, Dynamics of CM

Solution:

We will approximate the block’s position $x$ with the position $x_{\text{CM}}$ of the CM. Since both the block and the CM is within the cube of mass $M$, we know

To find $x_{\text{CM}}$, note that there is no net external force on the cube-block system so linear momentum is conserved. Since $p_{\text{tot}}=M_{\text{tot}}v_{\text{CM}}$, we know $v_{\text{CM}}$ is constant. We have

so at $t=60\,\mathrm{s}$,

Thus, the answer is B.