Final answer:
The correct verb to use is 'estoy', which means 'I am' in English.
Step-by-step explanation:
Yo estoy muy cansada hoy.
The correct verb to fill in the blank is estoy. In Spanish, the verb 'estar' is used to indicate temporary states or conditions, such as feeling tired. The verb 'ser', which means 'to be', is used for permanent characteristics or identities.
For example, you would use 'estoy' to say 'I am tired' and 'soy' to say 'I am a student'.
Learn more about Spanish verbs