Yes, that's the idea – it's grammatically correct, so I think it should be marked as a correct answer, even though the meaning is changing from plural to singular (I had to choose the correct answer from two or three alternatives). Given that both "autókat" and "autót" are correct in this case, I don't know if the sentence is given as some other exercice, for example to write it down after having listened to it, that's why I asked.
The problem is I don't know what kind of task you got this sentence for. All I see is a Hungarian sentence and an English counterpart which in this case are: "Én piros autókat keresek." Translation: I am looking for red cars. All I see is what you said and what the original is don't match up.
This is a good point.
The item is presented as an isolated sentence lacking any useful. While I recognize that the lesson is about plural accusative, items involving this distinction (singular vs. plural, in this case) should either make it clear somehow that the correct response may only be one or the other, or they should accept all grammatically correct responses. .
Either possibility seems better than the present ambiguous arrangement.