In the figure a 20 kg block is shown lying on a smooth horizontal plane, it is being pulled with a horizontal string that passes over a smooth, fixed pulley. A 5 kg block is kept on the 20 kg block and the coefficient of friction between the 5 kg block and the 20 kg block is 0.5. The coefficient of friction between the 20 kg block and the plane is 0.

When the 20 kg block is pulled with a force of 50 N, there is a force of friction between the 20 kg block and the 5 kg block. This is equal to `mu*N` = 0.5*5*10 = 24.5 N.

The acceleration of the 5 kg block is due to this force; the acceleration is 25/5 = 5 m/s^2.

**The correct answer is option D**

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