Sentence 15 is correct as is because the sentence has no grammatical errors with a sound sentence structure, is concise and clear about what it means to say- Polk, who was encouraged (idiom 'spurred on' means to urge or encourage) by Jackson, recognized that the two presidential candidates are not offering anything that the citizens actually want them to in their campaigns. Option A is the correct answer. The other options are incorrect as they do not fit into this context.