(A) which caused only a mild illness, they are immune from
(B) causing only a mild illness, they become immune from
(C) which causes only a mild illness, they are immune to
(D) causing only a mild illness, they became immune from
(E) which caused only a mild illness, they would become immune to

Splits
B&D out - Usage of verb-ing modifier incorrectly modifies the entire preceding clause. We need to modify only the preceeding noun entity i.e. cowpox. Correct usage of verb-ing modifier here would been sans the comma.
A out - Incorrect usage of verb-tense. The observation was in the past, the effect was with a time lag. Happened -- Would become
C out - Idiom problem. Immune from is incorrect.

E is the winner.

Request Expert Reply:
Hello Experts,
MartyTargetTestPrep, GMATNinja, GMATGuruNY, AjiteshArun, AndrewN
How do we know that 'the mild illness' has been occurred in the past (caused)? I mean: Why not it is 'causes'?
Thanks in Advance..

Either would work, actually, and the more natural choice is the present tense "causes."

Notice that the question writer used the less natural, and even debatably incorrect, past tense "caused" in the correct answer to make it less obviously correct.
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BrentGMATPrepNow EducationAisle mikemcgarry

Respected moderators
I have a different doubt in this , According to OG explanation an issue with using 'ing' modifier in option B and D has been pointed out . To quote
"The participle causing suggests that infecting subjects with cowpox caused a mild illness only in some of the cases - but this is not intended"

I am not able to understand the issue with using -ing modifier as it is a case where a participial modifier comes in the middle of sentence separated by commas, it should modify the noun ahead of it and it does so just fine

Looking forward to your help

PS - i have understood the other decision point and solved the problem correctly. It is just this point in OG ans key that is hurting me

The English physician Edward Jenner found that if experimental subjects were deliberately infected with cowpox, which caused only a mild illness, they are immune from smallpox.

(A) which caused only a mild illness, they are immune from
'are' is present tense, but this is about something in the past
'immune from' is not idiomatic, but 'immune to' is

(B) causing only a mild illness, they become immune from
'causing' modifies Edward Jenner...but surely Jenner doesn't cause illnesses
'immune from' is not idiomatic, but 'immune to' is

(C) which causes only a mild illness, they are immune to
'are' is present tense...incorrect

(D) causing only a mild illness, they became immune from
'causing' modifies Edward Jenner...but surely Jenner doesn't cause illnesses

(E) which caused only a mild illness, they would become immune to
Correct. Future in the past...'would become immune to'...so post infection they (the individuals who were infected) would become immune...
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