Multiattack (2014)
A creature that can make multiple attacks on its turn has the Multiattack action. A creature can't use Multiattack when making an opportunity attack, which must be a single melee attack.
Appendix D: Rule References / Rule
A creature that can make multiple attacks on its turn has the Multiattack action. A creature can't use Multiattack when making an opportunity attack, which must be a single melee attack.