Final answer:
A comma should be added after 'respected' in the sentence because it begins with a dependent clause, which needs to be separated from the main clause with a comma for clear understanding.
Step-by-step explanation:
The correct change that should be made to the sentence 'Because he was so respected the American people called him to duty.' is to add a comma after respected. When a sentence begins with a dependent clause like 'Because he was so respected,' a comma is needed to separate it from the main clause, which is 'the American people called him to duty.' This follows the punctuation rule that states a comma should be used after an introductory phrase to clearly distinguish between various components of a sentence.