The correct answer is the first option: "When adding a suffix that begins with a vowel to a root word that ends with a consonant, double the consonant before adding the suffix."
Explanation: this rule is done with monosyllabic words which have a vowel followed by a consonant at the end of the root word. It also applies to root words with the same characteristics mentioned above, but which have more than one syllable and the last syllable carries the main stress.
Examples:
Monosyllabic words Multisyllabic words with stress in the last syllable
Fur > Furry Prefer > Preferred
Blur > Blurred Occur > Occurring