Answer: "Pon las galletas a hornear por veinte minutos a 175°C".
Explanation: The verb that best completes the instruction in Spanish is "hornear", which can be translated into English as "to bake". "Hornear" is the correct verb because the writer is giving instructions about how to make cookies. Cookies are not fried, which is what the verb "freír" implies, nor boiled, which is what the verb "hervir" denotes. Moreover, the verb "amasar" (knead) cannot fill in the blank either because it does not collocate with the cooking temperature (175°).