¿Tú compras los huevos?
No. Tú los compras.
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¿El chef prueba las recetas?
No. El chef no las prueba.
To replace the accompanying noun, write a direct object. They can be replaced if it is masculine it is "lo" (singular) or "los" (plural) and if it is feminine it is "la" (singular) or "las" (plural).
In this case "los" is used because "los huevos" is a word that is masculine and plural, while "las recetas" is written "las" because the word is feminine and plural as well.