How long must an e-mail complaint be maintained according to FINRA rules?

The correct duration for maintaining an e-mail complaint according to FINRA rules is four years. This is consistent with FINRA's guidelines that set specific record-keeping requirements for various types of documents and communications, including complaints received through email. The four-year requirement is designed to ensure that firms keep adequate records for the purpose of regulatory oversight and to protect investors, providing a balance between the need for documentation and the ability for firms to manage archival records effectively.

In contrast, the other options suggest either an indefinite timeframe, which would be impractical, or a shorter period that's not in line with regulatory standards. Thus, maintaining complaints for four years aligns with industry norms and regulatory expectations for accountability and transparency.