Final answer:
The verb estar should be in the third-person singular form 'está' and the verb comprando indicates an ongoing action.
Step-by-step explanation:
The correct form of the verb estar to fill the blank in the sentence 'Ella está comprando un boleto' is option C. 'Está comprando'.
In this sentence, the subject 'Ella' (she) requires the third-person singular form of the verb estar, which is 'está.' The verb comprando (buying) is the progressive form, indicating an ongoing action.
Therefore, the correct answer is option C, 'está comprando.'
Learn more about Spanish verb conjugation