Although it kind of conveys the same information, the semantics are a little different. In the prompt the speaker is trying to determine the location of the chips, and thus Aia is used to say Are the chips on the plate as opposed to somewhere else like in the bag. If we change the English to "Are there chips on the plate?" then the speaker would be trying to verify the existence of chips on the plate. I think that is why it got marked wrong, though I would think it should be accepted since the meaning is so close.