Dono どの

There are two people (A san and B san) talking in a parking lot. A san points his car out and says B san that that is his car. But B san still doesn't know which one is his car. So B san asks him back and A san points his car out and says again.
I am also going to add one more sentence in each Example Q&A to answer your grammatical question.


(1)
A san: そのくるまは わたしのです。(SONO KURUMA WA WATASHI NO DESU) = That car is mine.

B san: (あなたの くるまは） どのくるまですか。(ANATA NO KURUMA WADONO KURUMA DESUKA) = Which car is it (your car)?

A san:(わたしの くるまは） そのくるまです。(WATASHI NO KURUMA WA SONO KURUMA DESU) = (My car) is that car.

(2)
A san: それは わたしの くるまです。(SORE WA WATASHI NO KURUMA DESU) = That is my car.

B san: (あなたの くるまは） どれですか。(ANATANO KURUMA WA DORE DESUKA) = Which one is it (your car)?

A san: (わたしの くるまは） それです。(WATASHI NO KURUMA WA SORE DESU) = (My car is) that one.


In the first example Q&A, the topic of the sentence (ANATA NO KURUMA /your car) is omitted since it is understood.

In the second example Q&A, the topic of the sentence (ANATA NO KURUMA /your car) could be omitted as in the first one, however, the くるまは (KURUMA WA) part is left and was not omitted for the purpose of grammatical explanation.
Both sentences have the topic/subject in the same place. It is just that one is hidden and the other one is not.