DIII
ST1: Donald trump may be the next president, and no one knows how possible is
THAT.
Is ST1 correct in GMAT, is "THAT" referencing correctly, if not what is the correct version of this sentence.
Can someone shed some light on this, thank you.
'That' (as well as 'this', 'these', and 'those') shouldn't be used as a pronoun by itself. It should be used with another word, as in @broall's example with 'that opinion'. Personally, I would use the phrase 'that outcome'.
In addition, there's a word order issue in your example sentence. It should read as follows:
Donald Trump may be the next president, and no one knows how possible that outcome is.