You should adopt . . . . . . . . orphan as . . . . . . . . heir.

Solution (By Examveda Team)

"An" is used before words that begin with a vowel sound, not just a vowel letter.

"Orphan" begins with a vowel sound, so the correct article is "an".

"Heir" starts with a silent "h", and the word sounds like it begins with a vowel ("air"), so it also takes "an".

Therefore, the correct sentence is:
You should adopt an orphan as an heir.

Hence, the correct option is D: an, an