Answer:
La camarera te lo sirvió.
Explanation:
In this case you must replace the direct object with a direct object pronoun.
Sirvió is the verb and "el plato de pasta con mariscos" is the complement of the verb which is the direct object.
La camarera te sirvió el plato de pasta con mariscos.
A typical mistake for a native speaker of English would be to place the direct object after the main verb "sirvió":
-La camarera te sirvió lo.
But this is wrong in Spanish because the pronoun is replacing the original phrase "el plato de pasta con mariscos" and It must go before the verb because place a direct object pronoun after the verb is ungrammatical in Spanish.
