Until the passage of the Piracy and Counterfeiting Amendments Act in 1982, a first-time charge of copyright infringement was merely a misdemeanor
charge, federal prosecutors being unlikely in pursuing criminal copyright infringers, while offenders were subject to relatively small penalties.
A. charge, federal prosecutors
being unlikely in pursuing criminal copyright infringers, while offenders
were Wrong. Parallelism problem.
B. charge, with federal prosecutors who
were unlikely to pursue criminal copyright infringers, offenders
being Wrong. Because a comma separates two clauses, "with" does not attach to the preceding clause thus it does not change meaning. Yet "with" does not make sense either.
Parallelism problem: see red part.
C. charge, federal prosecutors
unlikely to pursue criminal copyright infringers, while offenders
were Wrong. Parallelism problem.
D. charge; therefore, federal prosecutors
were unlikely in pursuing criminal copyright infringers and offenders
being Wrong. Parallelism problem.
E. charge; therefore, federal prosecutors
were unlikely to pursue criminal copyright infringers, and offenders
were Correct. The semicolon separates two clauses. The clause before a semicolon is the main clause, the clause after a semicolon is dependent clause.
In addition, "and" is also used to connect two independent clauses, which provide more information for the main clause (the one before a semicolon). Because two results (fed prosecutors were...... & offenders were......) are different and happen simultaneously, thus "and" is correct.
Hope it helps.