Final answer:
The correct verb to complete the sentence is 'estoy', which means 'I am' in Spanish.
Step-by-step explanation:
The correct verb to complete the sentence is estoy, which means 'I am' in Spanish. The reason why 'estoy' is the correct answer is that the sentence is talking about the state or condition of feeling sad today, which is a temporary emotion. So, we use the verb 'estar' to express temporary conditions or states.
Learn more about Spanish verbs