Answer: Juan construyó un castillo de arena.
Step-by-step explanation:
The sentence here is:
"Juan built a sandcastle"
Where the verb here is "Built", and the translation is "Construir"
if we want to write it in a preterite form that best suits the sentence, we should choose the perfect preterite (because the action has already been ended, Juan already built the sandcastle"
The conjugation would be:
Juan construyó un castillo de arena.