Which EtherChannel mode enables LACP only when the peer initiates it (i.e., only if an LACP device is detected)?

Passive mode enables LACP only when the peer initiates it. This means the interface will respond to LACP packets but will not actively send them unless requested. Desirable mode actively attempts to negotiate an EtherChannel using LACP. On mode forces the interface into an EtherChannel without LACP negotiation, so it won't form an EtherChannel with an LACP-enabled peer unless that peer is also in On mode. Auto and Active are PAgP (Port Aggregation Protocol) modes, not LACP modes.