An engineer is using Cisco Jabber. Instant messages are transported through which protocol?

SIP is a signaling protocol used for establishing voice and video sessions, not for transporting instant messages in Jabber.

XMPP is the open standard protocol that Cisco Jabber relies on for instant messaging and presence signaling. Cisco IM and Presence Service acts as an XMPP server, and Jabber clients communicate with it using XMPP stanzas. This protocol natively supports features such as contact lists, availability status, and message delivery.

H.323 is a legacy video and voice conferencing protocol and plays no role in Jabber instant messaging transport.

SCCP (Skinny Client Control Protocol) is a Cisco proprietary protocol used by IP phones for call signaling, not for IM transport.