1. Condition for combination of atomic orbitals (LCAO) to form molecular orbitals

For two atomic orbitals to combine effectively and form molecular orbitals, they must have:

• comparable energies,
• proper symmetry about the internuclear axis,
• appreciable overlap.

Here the internuclear axis is the $z$-axis.

2. Check each given pair

A. $2p_z$ and $2p_x$

• $2p_z$ is symmetric around the internuclear ($z$) axis and forms a $\sigma$-type orbital.
• $2p_x$ is oriented perpendicular to the $z$-axis and forms a $\pi$-type orbital.
• Since they have different symmetry about the internuclear axis, they cannot combine.

So, A is not possible.

B. $2s$ and $2p_x$

• $2s$ is spherically symmetric.
• $2p_x$ has $\pi$ symmetry with respect to the $z$-axis.
• Their symmetries do not match, so overlap cancels out.

So, B is not possible.

C. $3d_{xy}$ and $3d_{x^2-y^2}$

• Both are $d$ orbitals lying in the $xy$-plane.
• About the internuclear axis ($z$-axis), these do not have the same symmetry for effective overlap with each other in a homonuclear diatomic molecule.
• Their overlap integrates to zero by symmetry.

So, C is not possible.

D. $2s$ and $2p_z$

• Both are symmetric about the internuclear $z$-axis.
• $2s$ and $2p_z$ can both have $\sigma$ symmetry.
• Hence this linear combination is allowed and can form molecular orbitals.

So, D is possible.

E. $2p_z$ and $3d_{x^2-y^2}$

• $2p_z$ has $\sigma$ symmetry about the $z$-axis.
• $3d_{x^2-y^2}$ lies in the plane perpendicular to $z$ and does not match this symmetry.
• Hence they cannot combine effectively.

So, E is not possible.

3. Final conclusion

Only option D leads to formation of molecular orbitals.

Therefore, the correct choice is:

$\boxed{\text{D Only}}$

This corresponds to Option A.

4. Comparison with stored answer

Stored correct answer: A

Our derived answer: A

They match.