Which is correct, tougher, or more tough?

The correct answer is tougher.

When a word is monosyllabic, meaning that it has only one syllable, then its comparative form will be created by adding the suffix - er. Since the adjective tough has only one syllable, its comparative will be tougher. In case of words that have more than one syllable, such as beautiful, you would add more to create their comparative form: more beautiful instead of beautifuler.

From you choices which are tougher or more tough, the right answer is "tougher". More tough is wrong because the root word "tough" can be added by "er". The word "more" is only applicable for those words that cannot be added by "er".