In Canada's anti-spam legislation (CASL), one of the key requirements is that commercial electronic messages (CEMs) must provide recipients with a clear and readily accessible unsubscribe option. This means that recipients must have the ability to opt-out of receiving further electronic communications from the sender. This regulation is in place to protect individuals from receiving unwanted or spam emails. Therefore, option C accurately describes an aspect of CASL.
As for the other options: A. In CASL, the burden of proof lies with the sender to demonstrate that they have obtained proper consent for sending electronic messages. B. Pre-checked boxes for consent do not comply with CASL as it requires explicit opt-in consent. D. Consent under CASL can be implied in certain specified situations, but the timeframe is not limited to relationships within the past four years. E. While there are exemptions for existing business relationships under CASL, the law does not completely exempt CEMs from these relationships. F. CASL regulations apply to all industries, including real estate brokerages, and are not exempt from compliance.