Final answer:
The appropriate verb to fill in the blank is "estoy". It means "I am" and is used to indicate a temporary state or condition.
Step-by-step explanation:
The appropriate verb to fill in the blank is D. estoy. The sentence translates to "I am very tired today." The verb estar is used to indicate a temporary condition or state, such as feeling tired. In this case, the subject pronoun "yo" is followed by the verb "estoy", which means "I am".
Learn more about Verb conjugation