La sal de la mesa.
a. caíste
b. caí
c. cayeron
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)

this question is given using the conditional form. conditional tenses are is used when talking about hypothetical situations and probabilities. also, this is used when making polite requests. so comprarías is the conjugation of the verb comprar that matches the second person singular in the conditional. finally:

¿cuál comprarías tú?