"Which one is your car?"

Translation: Laquelle est ta voiture ?

[OloTheHero]

Whats wrong with "quelle est ton voiture"? Duolingo says to use suddenly automobile...

[relox84]

That's because you used the masculine "ton" instead of the feminine "ta" which is why it was wrong. Duolingo proposed "automobile" instead because it would be used with "ton" (automobile is a feminine noun too, but ton replaces ta before vowels and unsapirated h's).

[Sitesurf]

Remember: French possessives are adjectives and as such, they agree with the noun they modify. In addition, they do not give any indication on the owner's gender .

[Rekty]

And to add, you also messed up the gender of the word Quelle, which is feminine. If you use it with Ton, masculine, it wouldn't work as both need to have the same gender and number. You have to chose, either you keep it all masculine or all feminine, all singular or all plural!