I read a excellent explanation fr this question by someone on 1000 sc forum:
Analysis:
Verb Tense
- has been
- was
- was to be
- is necessary
- will be
It starts in the past and now in the present - Present Perfect Tense
Parallelism
- to develop and enforce
- for developing and enforcing
- in developing and enforcing
- to develop and enforce
- for developing and enforcing
Also, Subordinate clause
that Blacks could not be excluded outright from jury service
main clause
- that all juries must be
- of all juries being
- of all juries to be
- that all juries must be
- of all juries being
Idioms
- necessary for
- necessary to
B - out past tense, of juries being
C - out past tense, of all juries to be
E - out future tense, of all juries being
Between A and D
tense: has been vs. is
parallelism: both correct
Main vs. Subordinate: both correct
Idiom: necessary to both correct
A - past to present
D - present
A - Correct