Passage: The most distinctive phonological change separating Proto-Indo-Iranian from Proto-Indo-European is the collapse of the ablauting vowels *e, *o, *a into a single vowel, Proto-Indo-Iranian *a (but see Brugmann's law). Grassmann's law, Bartholomae's law, and the Ruki sound law were also complete in Proto-Indo-Iranian.

Question: What is most distinct difference?
collapse of the ablauting vowels *e, *o, *a into a single vowel