The correct answer is: C. XMPP
XMPP (Extensible Messaging and Presence Protocol) is an open standard for real-time communication. It is used by many instant messaging (IM) providers, including Google Talk, Facebook Messenger, and WhatsApp. XMPP is also used for other purposes, such as group chat, file transfer, and presence information.
SIP (Session Initiation Protocol) is a signaling protocol used for initiating, maintaining, and terminating multimedia sessions. It is used for voice over IP (VoIP), video conferencing, and instant messaging. SIP is not an open standard, but it is widely supported by vendors.
SIMPLE (Session Initiation Protocol for Instant Messaging and Presence Leveraging Extensions) is an extension to SIP that is used for instant messaging and presence. It is not as widely supported as SIP, but it is used by some IM providers, such as Yahoo! Messenger.
IMPS (Internet Message Access Protocol) is a protocol used for accessing email. It is not used for instant messaging.
In conclusion, XMPP is the protocol that is adopted for interoperability by top three IM service providers.