ST:AB::YZ: ? complete the series
We see that the pattern or the series followed in this question is that we have S as the first character, and A as the first character after the colon. We also observe that Y is 6 characters away from S, which effectively means that the the first character after the last colon will be 6th character after A, which is G, so the series will be the following:
ST:AB :: YZ:GH