De repente el oso al niño. a. vio b. veía c. vieron

Let's analyze the options:

a. vio - it's a conjugation that matches él/ella/usted in the preterite.

b. veía - It's a conjugation that matches él/ella/usted in the imperfect.

c. vieron - It's a conjugation that matches ellos/ustedes in the preterite.

The group of words de repente means suddenly and this tells us that the sentence needs to be in the preterite. Since el oso matches the third person singular, then the only option that meets the requirements is a. vio. Finally:

De repente el oso vio al niño