802.3af is the correct answer because it is the PoE standard that provides up to 15.4 watts of power per port, which is suitable for devices like VoIP phones, wireless access points, and security cameras. 802.3at, also known as PoE+, provides up to 30 watts of power per port, which is more than needed for most VoIP phones and could indicate a more costly infrastructure than necessary. The other options, 802.3ad and 802.3bt, are incorrect. 802.3ad is actually a standard for link aggregation and not related to PoE, and 802.3bt is an extension to the original PoE standards, supporting up to 60 or 100 watts per port, which is also excessive for standard VoIP phone requirements.