... and I think I was wrong yesterday night (maybe it was already too late to think clearly :), because the molecule Y is obviously 1,3-Cyclohexadiene, thus of course it need not to be sketched wrongly.
In this case the red color doesn't point out at any mistake, but that it is not a single bond.
It is possible that we should count the double bonds, or otherwise all bonds which are not single bonds (all multiple bonds).
... and I think I was wrong yesterday night (maybe it was already too late to think clearly :), because the molecule Y is obviously 1,3-Cyclohexadiene, thus of course it need not to be sketched wrongly.
In this case the red color doesn't point out at any mistake, but that it is not a single bond.
It is possible that we should count the double bonds, or otherwise all bonds which are not single bonds (all multiple bonds).
Hot
The original image is here. The red highlight is there for teaching purposes, and likely irrelevant when taken out of context (in this puzzle).
Little hint: the amount and type of bond counts