Easy
Inside the box, there was .............................. envelop.
Explanation
Explanation
We use 'an' before words that begin with a vowel sound (a, e, i, o, u). The word 'envelop(e)' begins with the vowel sound /e/, so the correct article is 'an'. The answer is option (2).
PRO TIP
Re-check your working by substituting the answer back into the original question.