Concept:A one-word substitution question asks for a single word that expresses the given phrase.Explanation:“A person or thing that acts as a defense” means something that gives protection or support.The word “bulwark” means a defensive wall or a protective person or thing.“Bumble” means to act or speak clumsily.“Bullion” means gold or silver in bulk form.“Bulge” means a swollen or protruding part.So, the correct substitute is “bulwark”.Answer:Bulwark.