What is the purpose of the TCP SYN flag?

What is the purpose of the TCP SYN flag?

A.
to sequence each byte of data in a TCP connection

B.
to synchronize the initial sequence number contained in the Sequence Number header field with
the other end of the connection

C.
to acknowledge outstanding data relative to the byte count contained in the Sequence Number
header field

D.
to sequence each byte of data in a TCP connection relative to the byte count contained in the
Sequence Number header field