Which combinations of orbitals lead to pi bonds according to Valence Bond Theory?
Here are the requirements for a $\pi$ bond:

Each contributing orbital has to have exactly one nodal plane containing the bond axis.  There can be other nodal planes that might cut through the axis, but don't contain it so those planes don't count.  Two nodal planes containing the axis, as with a $d_{xy}$ orbital, also fail because that's too many.

The nodal planes identified in (1) that contain the bond axis must line up, so they end up as a single nodal plane going through the bond.  Draw, let us say, the $p_x-p_x$ pair, which is easy to see it's a $\pi$ bond, to see what I mean.


If you sketch the orbital combinations for each of the nine given cases you see immediately that three fail test (1) (the $s$ orbital is no-go for a nodal plane, and $p_z$ has its nodal plane oriented wrong), but the others pass both tests (1) and (2) and they are the six combinations identified by the book.
The hidden lesson here is that $d$ orbitals can form $\pi$ bonds too.  You will see them do so when you study transition-metal complexes.