How do two BGP peers signal their capabilities to each other?

A.
They include their capabilities in OPEN messages.

B.
They send UPDATE messages with their capabilities after the session is established.

C.
They send CAPABILITIES messages before they send OPEN messages.

D.
The network operator must manually configure the capabilities on both BGP peers.

Explanation:
BGP Open Message
An OPEN message is sent after the TCP three-way handshake is completed. The BGP OPEN message is used to open a BGP peering session. The OPEN message contains information about the BGP neighbor initiating the session, and information about supported and negotiated options including:
BGP Version number
AS Number
Hold Down Time value
BGP Identifier
Optional Parameters
Optional Parameters length (0-255 in bytes)
Open Message Format

Version (8-bits)
The BGP OPEN message’s Version field contains 8-bits that represent an unsigned integer that indicates the version number of BGP, currently this field is set to 00000100 (decimal 4).
My AS Number (16-bits)
The BGP OPEN message’s AS number field contains 16-bits that contains the AS number of the BGP routing instance that transmitted the OPEN message.
Hold Down Timer (16-bits)
The BGP OPEN message’s Hold Down Timer field is a 16-bit field containing the proposed number of seconds between BGP KEEPALIVE and UPDATE messages. The Hold Down Timer must be either zero, or at least 3 seconds and defaults to three times the KEEPALIVE message interval.
BGP Identifier (32-bits)
The BGP OPEN Message’s BGP Identifier is a 32-bit integer that indicates the ID of the sender of the BGP OPEN message and is equal to the IP address that is assigned to the device. This ID is used for all BGP peering sessions on that router.
Optional Parameters Length (8-bits)
The BGP OPEN Message’s Optional Parameters Length is an 8-bit field that indicates the number of bytes in the Optional Parameters section of the BGP OPEN Message.
Optional Parameters
The BGP OPEN Message’s Optional Parameters contains all optional parameters for BGP sessions.