How many bits is a acknowledgement number?

Transmission control protocol accepts data from a data stream, divides it into chunks, and
adds a TCP header creating a TCP segment.
The TCP header is the first 24 bytes of a TCP segment that contains the parameters and
state of an end-to-end TCP socket. It is used to track the state of communication between
two TCP endpoints.
For a connection to be established or initialized, the two hosts must synchronize. The
synchronization requires each side to send its own initial sequence number and to receive a
confirmation of exchange in an acknowledgment (ACK) from the other side The below
diagram shows the TCP Header format:

How many bits is a acknowledgement number?

A.
16 bits

B.
32 bits

C.
8 bits

D.
24 bits

Explanation:

Reference:
http://en.wikipedia.org/wiki/Transmission_Control_Protocol(acknowledgement number)