The species which can best serve as an initiator for the cationic polymerization is:

Cationic polymerization is initiated by acids or Lewis acids. ${\text{AlCl}}_3$ is a strong Lewis acid that can generate a carbocation in the presence of a co-catalyst like ${\text{H}}_2\text{O}$ or $\text{HCl}$, making it an ideal initiator. ${\text{LiAlH}}_4$ and $\text{BuLi}$ are used for anionic polymerization.