Concept:The phrase “to bear the brunt of” means to endure the main force or worst part of something.Explanation:The sentence is “The infantry has _________ a battle.”The blank needs an infinitive phrase after “has”.“To bear the brunt of” means to face the main impact.So the completed sentence is: “The infantry has to bear the brunt of a battle.”Other options do not fit grammatically or contextually.Answer:To bear the brunt of.