If the hydrangea blooms in late summer it is a panicled hydrangea.

Inferences:
-- It can be inferred from the argument that classification based on 'grown for florists' supersedes that based on 'the blooming season'.
-- These seem to be two different classification systems since the basis for classification is not 'grown for florists versus not grown for florists', or 'this blooming season vs that blooming season'. Thus, there can be overlap.
-- If the hydrangea is grown for florists, it is big leaf hydrangea, irrespective of the blooming season.

The argument above relies on which of the following to reach its conclusion?

A) The panicled hydrangea has no features in common with the bigleaf hydrangea.
We don't know that. It might have some features in common.

B) If a hydrangea is panicled, it is not bigleaf.
It can be inferred from the argument that 'grown for florists' supersedes the blooming season. So, if we have identified a hydrangea as 'bigleaf', it can't be 'panicled'. And if it is 'panicled hydrangea' then we did not feel the need to call it 'bigleaf hydrangea' in the first place (the reason for which could have been that it was not grown for florists). CORRECT.

C) No bigleaf hydrangea blooms in the late summer.
We only know about panicled hydrangea blooming in late summer. Nothing to comment on big leaf's blooming.

D) All panicled hydrangea blooms in the late summer.
The argument says in first sentence 'If the hydrangea blooms in late summer it is a panicled hydrangea'. It doesn't say 'Only If the hydrangea blooms in late summer it is a panicled hydrangea'. May be we have panicled hydrangea that bloom in some other season(s) too.

E) Of the hydrangeas used by florists, none blooms in the late summer.
'used by' is not the same as 'grown for'

It can be simplified this way:

Premise:
Summer --> Panicled
Florists --> Bigleaf

Conclusion:
Florists --> Not Panicled

Pre-thinking:
The direct assumption that leads to that conclusion is that Bigleaf <---> Not Panicled

Options:
A) it is talking about features, which are not mentioned --> out of scope

B) If a hydrangea is panicled, it is not bigleaf : Panicled ----> Not Bigleaf

C) No bigleaf hydrangea blooms in the late summer: Summer --> Not Bigleaf

D) All panicled hydrangea blooms in the late summer: Summer --> Panicled and Summer --> Not Bigleaf

E) Of the hydrangeas used by florists, none blooms in the late summer: Bigleaf (used by florists) --> Not Summer

B is the only option that defines the link between the Bigleaf & Panicled hydrangea

IMO, this question has some grey area in its options.
There is nowhere mentioned that both these varieties cannot overlap in any situation.
Also, option C and E provide a much better assumption to the statement.

Also Option B says, If a hydrangea is panicled, it is not bigleaf. But in order to get our assumption, we need a sentence saying that If hydrangea is Bigleaf, it is not panicled.

Looking for a satisfactory explanation for this question.

The explanation in the book says “but if a panicled hydrangea cannot be a big leaf, then a big leaf hydrangea cannot be panicled”

I agree we cannot assume this so the question is invalid

Posted from my mobile device

"This hydrangea is grown for florists and is therefore not panicled." <-- This statement relies on the assumption that there is no overlap (hydrangea cannot be both). If this assumption was not made, the statement could not be accurate, therefore B must be correct. If it's one, then it cannot be the other.