Here are the answers to your questions based on the information provided:
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"el cuaderno" (the notebook) - correct option for "O un cuaderno O la escuela O el cuaderno O unos cuadernos"
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"el chico" (the boy) - correct option for "O un sofá O el chico O la pluma O el cuaderno"
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"Change the Z to a C and add es" - this is the correct method for making a noun plural that ends in a Z.
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"la pizarra" (the whiteboard) - the correct definite article for "pizarra" is "la" as it is a feminine noun.