1-In case of slow start in every RTT window size becomes double or we can say for each acknowledgement window size increases by 1 MSS(maximum segment size). In slow start phase window size increases exponentially.
2-when it reaches threshold slow start phase ends up ,so after that congestion avoidance phase will start so for every RTT window size increases by 1 MSS.
so the given statement is true.(As question has been asked in slow start phase)