Question

el campo ------- silencioso mientras la oveja caminaba sola.
ha sido
era
será
fue

Explanation:

The sentence describes an ongoing past action ("la oveja caminaba sola" uses imperfect past tense, meaning the sheep was walking alone continuously in the past). To match this, we need the imperfect past tense of "ser" for the field being silent during that same ongoing past time. "Era" is the imperfect past form of "ser" for third-person singular, which fits the context of a continuous past state occurring alongside another continuous past action.

Answer:

era