The correct option is D Increases exponentially
In the slow-start (additive) phase of the TCP congestion control algorithm, the size of congestion window increases exponentially TCP acts the window size as follows allowed - window = min (receiver_advertisement, congestion -window)
In slow-start (additive) phase whenever starting traffic on a new connection or increasing traffic after a period of congestion , start the congestion window at the size of single segment and increases the congestion window by one segment each time an acknowledgement arrives.