The base word of uncomfortable is

D.when the base word ends with consonant + vowel + consonant and the suffix begins with a vowel

Explanation:

If you study English, you have heard of words ending in "consonant + vowel + consonant" (or CVC). These endings have a simple half rule: when the suffix begins with a vowel, the last consonant of the base word must be duplicated. Example: The base word is "stop" and the word "ed", when you add the base word with the suffix, the last letter "p" of the base word must be duplicated, becoming "stopped".