A man of mass $m_0$ is standing on a long plank of mass $M$  The man starts to run on plank with velocity $u_0$with respect to plank. 

 

(1) is wrong because $V_{cm}$is always $\text{'}0\text{'}$ because $\underset{ }{\sum F_{ext}= 0 and hence \left(a_{cm}\right) = 0}$

$\left(2\right)$ is wrong because work done by static friction on the system is always zero.

$\left(3\right)$is right because displacement of the plank is in the direction of frictional force acting on it .

$\left(4\right)$ is correct because internal energy of man converted into KE of the system