Concept:Here we need the
opposite (antonym) of the underlined word.
Colloquial language is the informal, everyday language of ordinary conversation, for example
gonna or
chill out.
Solution:The opposite of informal, conversational language is
formal language: proper, official and suited to serious writing or speeches.
Informal means the same as colloquial (a synonym, not an opposite).
Archaic means old-fashioned and
modern means current; they describe age, not formality.
Key point: Questions 26 to 30 of this paper ask for the word opposite in meaning. The old key gave (A) informal, which is the synonym, so it is wrong; the antonym is (C) formal.
Answer:Option C (formal)