A. Formal usage of a word is considered colloquial.
-quite the contrary, formal and colloquial can be seen in oppostion to each
other!!
B. Foreign words are not included in English dictionaries.
-but they are! Specifically English words of foreign origin, such as Bratwurst.
C. A syllable is a unit of sound.
-this is true! correct answer!
D. A dictionary's word entry offers a fixed, permanent meaning.
-this is not true! meaning changes very often actually, for example "mouse" used to refer only to an animal!