Original sentence-
Cleopatra’s extraordinary education was the result of many factors, not the least of
which was her proximity and access to the library of Alexandria, indubitably the greatest library in the world at the time.
The first part is a complete independent clause.
Rewriting the 2nd part -
Her proximity and access to the library of Alexandria, indubitably the greatest library in the world at the time, was not the least of the factors .(replacing - which)The above is an independent clause again (with connected ideas via "which").
Semicolons Connect Related Independent Clauses. Thus, we need Semicolon here.
(A) factors, not the least of which was her proximity and access to
Two independent clauses connected by comma - incorrect(B) factors; not the less of which was her proximity and access to
Correct(C) factors; not the least of which being her proximity to and access to
Being should only be used to explain state.
Her proximity can't greatest library (the non essential modifier ) (D) factors not the least of which was her proximity and access to
No semicolon. The construction is not correct. (E) factors. Not the least of which being her proximity to and access to
The two clauses are independent but related. Related clauses better be connected with semicolon and not a full stop.IMO
B