Right answer:
aprendió
Ayer Carlos aprendió a esquiar
Aprendió is the conjugation of the verb aprender for the third person singular in the preterite tense. The preterite tense is one of two simple past tenses. It is used to describe actions that took place or were completed at a certain point in the past. We know this sentence is given in the past because of the word ayer whose translation is yesterday.