Explanation:
In order to determine which orbitals will not form a sigma bond after overlapping, we need to consider the characteristics of sigma bonds and the nature of the orbitals involved.

Characteristics of sigma bonds:
- Sigma bonds are formed by head-on overlap of atomic orbitals.
- They are the strongest type of covalent bonds.
- They allow for free rotation around the bond axis.

Nature of the orbitals:
- S orbitals are spherical in shape and have maximum electron density at the nucleus.
- P orbitals are dumbbell-shaped and have two regions of maximum electron density on opposite sides of the nucleus.

Analysis of the options:
a) s-orbital and 5-orbital:
- This combination involves an s orbital and a higher energy orbital, such as a 5 orbital (which could be a d or f orbital).
- Since the s orbital has maximum electron density at the nucleus, it can form a sigma bond with the 5 orbital.
- Therefore, this combination can form a sigma bond.

b) s-orbital and pz-orbital:
- This combination involves an s orbital and a pz orbital.
- The s orbital has maximum electron density at the nucleus, while the pz orbital has maximum electron density along the axis.
- When the s orbital and pz orbital overlap, they form a sigma bond.
- Therefore, this combination can form a sigma bond.

c) pz-orbital and pz-orbital:
- This combination involves two pz orbitals.
- Both pz orbitals have maximum electron density along the same axis.
- When the two pz orbitals overlap, they form a sigma bond.
- Therefore, this combination can form a sigma bond.

d) px-orbital and px-orbital:
- This combination involves two px orbitals.
- The px orbitals have maximum electron density on opposite sides of the nucleus.
- When the two px orbitals overlap, they do not form a head-on overlap required for the formation of a sigma bond.
- Instead, they form a pi bond, which is a type of bond that is formed by the side-to-side overlap of two parallel orbitals.
- Therefore, this combination does not form a sigma bond.

Conclusion:
The correct answer is option 'D', as px orbitals do not form a sigma bond after overlapping.