Got this question wrong so here's the breakdown of why C is wrong and A is correct.
The passage states, "in 1948 he became a member of an impromptu musical think tank and this group "not only slowed down tempos and featured ensemble playing as much as or even more than solos..."
What can we infer from this?
A) It was characterized by rapid tempos and an emphasis on solo playing.
How do we know this?
Well if he slowed it down then it must have previously been faster and if there was at least if not more ensemble playing than solo after 1948 ( as a result of his changes ) that means that their was previously a greater proportion of solos which is precisely what answer choice A states.
Lets look at the rest and see why they are not supported by the passage:
B) It was perhaps more balanced but nothing in the text indicates that it was "equally" balanced and for that reason this answer cannot be correct.
C) It does say in the text that Davis eventually rebelled against the restrictive jazz styles but that wasn't until the late 1950s (line 20-24) and so this gets the time period wrong and is thus incorrect.
D) Jazz purists are mentioned in line 34 in and 46 but there is no indication they believed there is only one authentic style of jazz. They didn't like some of the changes Davis brought about but that doesn't support the claim they believed their is only one authentic jazz style which constitutes the totality of jazz. Maybe that is true but unfortunately this passage cannot bear the weight of such a claim based upon the information we have at our disposal and therefore we can safely eliminate D.
E) NYC Jazz clubs? What? We know about an apartment in which they gathered. This is out of no where and can easily be discarded.
Hope this helps!