We should not pick the choice in SC because something "sounds" good or something is what people "generally" say.
This problem is an example to demonstrate the same.
In day-to-day world, we can say : <plural subject> are A <something>
However, if the subject is plural, it can NOT be "a/an" ,A) women nurses, familiar with various surgical procedures, were a help in treatingWe can eliminate it.
This and These are demonstrative pronouns and can't stand alone (should be followed by nouns).
Examples - This boy, that problem etc...B) women nurses were familiar with various surgical procedures and this helped them to treatWhat does "this" stand for?
If I write the sentence as -
women nurses were familiar with various surgical procedures and
this familiarityhelped them to treat...
The above will be right.
Now, look at the choices D and E (both starting with modifier phrases) -
D.
During the early years of the American Civil War, when the Union Army provided one assistant surgeon to every regiment of a thousand men,having intimate knowledge of various surgical procedures, women nurses helped in the treatment.
Here, "having intimate knowledge of various surgical procedures" can ambiguously modify "men" and not "women"
E.
During the early years of the American Civil War, when the Union Army provided one assistant surgeon to every regiment of a thousand men,familiar with various surgical procedures, women nurses helped to treat.
Here "familiar with various surgical procedures" can ambiguously modify "men" and not "women"
Choice C fixes all the above issue and is correct.
C) women nurses, familiar with various surgical procedures, helped to treatIMO
C