Puritan fanatics brought to civil and military affairs a coolness of

Puritan fanatics brought to civil and military affairs a coolness of judgment and mutability of purpose that some writers have though inconsistent with their religious zeal, but which was in fact a natural outgrowth of it.
(A) but which was in fact a natural outgrowth of it
(B) but which were in fact a natural outgrowth of it
(C) but which were in fact natural outgrowths of it
(D) but it was in fact a natural outgrowth of them
(E) which was in fact a natural outgrowth of it

Question talks about 2 things - 'a coolness of judgment' & 'mutability of purpose' hence we need were to address both!
Leaves us only with B & C...
Since 2 things have said to grow naturally out of the religious zeal (the pronoun 'it'), it should be plural again. Hence.. C is correct...

key is to consider 'a coolness of judgment and mutability of purpose that some writers have though inconsistent with their religious zeal' as one relative clause and 'but which was in fact a natural outgrowth of it. (to be corrected part)' as a subordinate clause which would modify the relative clause. Hence it needs to modify both the things and not one.