The correct answer is: " unos escritorios " .
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Note: We are ask to find the plural form of "un escritorio" ; which takes the "indefinite article" ; which in this case is: "un escritorio" (that is: "a desk").
This is a masculine singular noun, so the indefinite article is "un".
The plural form would be: "unos escritorios" ; (that is: "some desks").
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