Final answer:
The correct use of the "double the consonant" rule in the given options is 'vomit-vomitted', following the CVC pattern and stress criteria when adding a vowel-starting suffix.
Step-by-step explanation:
The "double the consonant" rule is correctly demonstrated in option b. vomit-vomitted. This is because the word vomit has one vowel sound before the final consonant, ends with a CVC (consonant-vowel-consonant) pattern, and the stress is on the first syllable.
When a word like this has a suffix starting with a vowel added to it, in this case, the past tense suffix '-ed', you double the final consonant. Therefore, the correct form is 'vomitted'.