A language that can be described by a context$-$free grammar $($CFG$)$ but not by a regular grammar is:

The set of strings where every $\text{'}a\text{'}$ is followed immediately by a $\text{'}b\text{'}$

The set of strings with balanced parentheses

The set of all strings over the alphabet $\{a,b\}$

The set of strings with an equal number of $\text{'}$a$\text{'}$s and $\text{'}$b$\text{'}$s