Practice Test 3

Yes; because $A$ and $B$ are $2\times 2$, all $2\times 2$ matrices commute under multiplication.

No; matrix addition is not commutative, so $AB\ne BA$.

No; matrix multiplication is not commutative in general, so $AB\ne BA$ is possible even for square matrices.

Yes; because $AI=IA$ for the identity matrix $I$, all matrices commute with each other.