Concept:The phrase “bear the brunt of” means to face the main force or worst part of something.Explanation:The sentence needs a phrase that fits before “a battle.”“To bear the brunt of” changes to “has borne the brunt of” in the present perfect tense.It correctly means the infantry suffered the main impact of the battle.The other idioms do not fit grammatically or contextually.Answer:Option A: To bear the brunt of