Final answer:
Over half of all parolees who had their parole revoked and were returned to prison faced revocation because of new criminal offenses committed.
Step-by-step explanation:
The question is asking about the main reasons for the revocation of parole. Over half of all parolees who had their parole revoked and were returned to prison faced revocation because of new criminal offenses committed, which is option a. This means that the parolees committed new crimes while on parole, violating the conditions of their release.