Why cant we use baik rather than bagus?
The "baik" vs. "bagus" distinction is not easy. Why is it "bagus" here, while it is "teknik menulis yang baik" in another sentence in this skill? I feel that an example and a technique are two very similar things.
why is it 'bukan' and not 'tidak contoh yang baik'?
Because "contoh" is a noun and is negated by "bukan". See https://forum.duolingo.com/comment/31291121
But could we say "Ini contoh yang tidak baik" ? Probably the item in question IS in fact an example, just not a good one.
Ini bukan contoh yang baik. My answer is true.
why can't we use "benar" in place of "bagus" here?
Ini bukan contoh yang benar = This is not a correct example