The correct option is
D A
Articles 'a', 'an' and 'the' indicate that a noun will follow. They modify the noun, by telling us whether the noun is specific or non-specific, known or unknown.
'A' and 'an' are indefinite articles used when the noun is non-specific. The noun is mentioned for the first time and 'a' and 'an' help explain their context to the reader. 'A' is used when the noun begins with a consonant (bcdefgh...z) and 'an' is used when the noun begins with a vowel (aeiou).
'The' is used when the noun has been mentioned before (a specific noun), or the identity of that noun is known to the reader.
Option D: In the given sentence, the noun 'blacksmith' is mentioned for the first time. It begins with the consonant 'b'. Hence option D is correct.
Option A: 'His' expresses possession of something. There is no relation expressed between 'he' and the 'blacksmith'. Hence the possessive pronoun cannot be used here.
Option B: 'An' is used when the noun begins with a vowel, whereas 'blacksmith' begins with a consonant (b).
Option C: 'The' is a definite article used when the noun is specific or known. The 'blacksmith' is not a specific one, hence 'the' won't be used here.
So, options A, B and C are incorrect.