Right answer:
d. cayó
Cayó is the conjugation of the verb caer that matches the third person singular in the preterite tense. This is an irregular verb, hence you must learn by heart its conjugations. Recall that 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. Finally:
La sal cayó de la mesa (The salt fell off the table)