n, l and m values of the 2p_z orbital are:
 

n is principle quantum number, for 2p_z it is 2. As the shell number is 2.

l is azimuthal quantum number, for 2p_z it is 1. 

As the value of azimuthal quantum number for p sub shell is always 1.

m is magnetic quantum number, for 2p_z it is 0.

As the orbital angular momentum has no component along z - axis and have it must become zero and therefore to make l=0, we must take m=0.