Is the difference between gennych and gynnoch regional?
gynnoch chi is the standard form. Variations include gennych chi, ganddoch chi. Stick with gynnoch chi unless you have a particular reason not to.
What does "wyt ti'n cael car?" Translate to?
Most likely: 'Are you getting a car?'
If it was in a sentence such as 'Wyt ti'n cael mynd yno mewn car?' then cael has a sense of being allowed to do something:
What is wrong with the answer "Oes car gyda chi?"
Nothing, it's fine. It is one of the accepted answers.
You have two selections exacrly the same, but only one seems to be correct
Just checked, and there was a typo in one of the answers - now fixed. Thank you for letting us know.