Final answer:
The correct plural form is sergeants-at-arms.
Step-by-step explanation:
The correct plural form to complete the sentence is 2) sergeants-at-arms. In the context of the sentence, King Richard I of England, also known as Richard the Lionheart, assigned sergeants-at-arms. The term 'sergeants-at-arms' refers to a group of individuals who serve as armed attendants in a royal or ceremonial setting.