An indefinite article is used when we are talking generally about something whereas a definite article is used when we are talking about something specific.
The indefinite articles are 'a' and 'an'. An is used before a word starting with a vowel or vowel sound.
A definite article is an article that tells the uniqueness of something.The is the only definite article.
As the blank is followed by the word 'open-fire' which starts with the vowel 'o' and is a noun complement, it will take 'an'.
'There's an open-fire in the living room.'
The correct answer is option C) An,