I think the answer may vary according to the nature of acknowledgement .
If acknowledgement is always cumulative , then it's congestion avoidance , since every RTT window size increases by 1 only.
If acknowledgement comes for every segment , then it is slow start phase , because let's say current window size is 4 , so acknowledgement comes for every segment , and window size increases per ack. So after 4 ack's , new window size is 8 . Thus per RTT window size increases exponentially.
Correct me if i went wrong somewhere.