Which one of the following is NOT an example of an expanded octate?

In $\text{PCl}_5$, the central atom phosphorus forms $5$ single bonds, having $10$ valence electrons around it, which is an expanded octet.

In $\text{H}_2\text{SO}_4$, the central atom sulfur forms $6$ bonds ($2$ double bonds and $2$ single bonds), having $12$ valence electrons around it, which is an expanded octet.

In $\text{SF}_6$, the central atom sulfur forms $6$ single bonds, having $12$ valence electrons around it, which is an expanded octet.

In $\text{CH}_4$, the central atom carbon forms $4$ single bonds, having exactly $8$ valence electrons around it. Carbon belongs to the second period and lacks vacant d-orbitals, so it cannot expand its octet.

Thus, $\text{CH}_4$ is not an example of an expanded octet.

Answer: $\text{CH}_4$