"Вы думаете , это тарелки?"

Translation: Do you think these are plates?

[Hannanv]

Shouldn't it be "эти тарелки"?

[slycelote]

When the word это is used as a subject, it doesn't decline. When it's used as an attribute, it declines as the corresponding noun: These plates are clean - Эти тарелки чистые.