Final answer:
The correct preterite verb to fill in the blank is 'fui'.
Step-by-step explanation:
The correct preterite verb to fill in the blank is fui.
'Fui' is the first-person singular form of the verb 'ir' in the preterite tense, which means 'to go'. In this case, it shows that the person went alpinismo (mountaineering) with Juan last Sunday.
Learn more about Spanish verb conjugation