The correct option is
B One
Option B: 'One' as an adjective modifies the noun 'side'. 'One' indicates 'a certain thing', like 'a certain side'.
The meaning is relevant to the given statement and hence option B is correct.
Option A: 'An' comes before nouns that begin with vowels (a, e, i, o, u). But the noun 'side' begins with a consonant 's'.
Option C: 'His' refers to Nitin. The passage does not suggest that the old woman are on the same side. Hence this option does not help form a clear meaning.
Option D: 'Other' makes the noun 'side' specific. In that case, we require the definite article 'the', without which the sentence formed is incomplete.
Hence options A, C and D are incorrect.