Is it ... armchair?

Solution:

To correctly fill in the blank, we need to consider the word that follows. The word is 'armchair', which begins with the vowel sound /ɑː/. When a word begins with a vowel sound, the indefinite article 'an' is used.

1. The word 'armchair' starts with a vowel sound.
2. Therefore, the correct article to use is 'an'.

Answer: an