Final answer:
The correct answer is C. estaba.
Step-by-step explanation:
The correct answer is C. estaba.
The sentence is in the past tense, and the imperfect form of the verb estar is used to describe ongoing or habitual actions in the past. In this case, the sentence is describing a state or condition of being tired that lasted for a period of time in the past. For example, you could say 'Yesterday, my mother estaba muy cansada.' ('Yesterday, my mother was very tired.')
Learn more about imperfect form of the verb estar