Final answer:
The correction for the sentence should be 'hot, she could' to eliminate the double negative and add proper punctuation.
Step-by-step explanation:
The sentence in question, 'The weather was so hot, she couldn't hardly breathe outside.' has a double negative issue that should be corrected for clarity and grammatical accuracy. The correct answer is b) hot, she could which fixes the double negative ('couldn't hardly') and adds a comma for proper syntax, changing the sentence to 'The weather was so hot, she could hardly breathe outside.'