In slow start phase, reception of each ACK adds 1 MSS to cwnd.
Generally, if you send, say 4 packets (cwnd = 4) then in the time approximately equal to an RTT you'll get 4 ACKs in return. These 4 ACKs would add a total of 4 MSS in the cwnd.
Hence approximately we double our cwnd every RTT in Slow Start Phase. Statement iv is True.
Statement i is wrong as cwnd increases by 1 MSS on every ACK received. Hence, Statement ii is also wrong.
Statement iii could be true with slight modifications.
If every RTT successfully delivers you an ACK, then in the time somewhat equal to RTT, cwnd will get doubled. "Somewhat equal" because we'll have to wait for ACKs to get delivered back-to-back.