Q2.4

The set of all bijective functions on a finite set forms a group under function composition

The set $\{1, 2, \dots p-1\}$ forms a group under multiplication mod $p$, where $p$ is a prime number

The set of all strings over a finite alphabet forms a group under concatenation

A subset $S \neq \emptyset$ of $G$ is a subgroup of the group $\langle G, * \rangle$ if and only if for any pair of elements $a, b \in S, a * b^{-1} \in S$