To determine which pair of orbitals form a bond when the y-axis is taken as the internuclear axis, we need to consider the symmetry of the orbitals and their overlap.

The y-axis is the vertical axis, so we need to consider orbitals that have symmetry along this axis.

(1) px px:
The px orbital has a nodal plane along the y-axis, meaning it has no symmetry along this axis. Therefore, the px orbital does not overlap and form a bond when the y-axis is the internuclear axis.

(2) Pz Pz:
The pz orbital has symmetry along the y-axis, as it is oriented along this axis. When two pz orbitals overlap, they form a sigma bond along the y-axis. Therefore, the pz pz orbital pair does form a bond when the y-axis is the internuclear axis.

(3) s px:
The s orbital has spherical symmetry and does not have any symmetry along the y-axis. The px orbital, on the other hand, has a nodal plane along the y-axis. When these two orbitals overlap, they do not align properly and do not form a bond along the y-axis. Therefore, the s px orbital pair does not form a bond when the y-axis is the internuclear axis.

(4) s py:
Similar to the previous case, the s orbital does not have any symmetry along the y-axis. The py orbital, however, has symmetry along the y-axis. When these two orbitals overlap, they do not align properly and do not form a bond along the y-axis. Therefore, the s py orbital pair does not form a bond when the y-axis is the internuclear axis.

In summary, the pair of orbitals that form a bond when the y-axis is taken as the internuclear axis is pz pz.

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