Final answer:
The appropriate present tense form to fill in the blank is "está."
Step-by-step explanation:
The appropriate present tense form to fill in the blank is está. The sentence translates to "Mrs. Alvarez's class is very difficult." In Spanish, the verb estar is used to express temporary states or conditions, such as how difficult the class is, while the verb ser is used for permanent characteristics or identities.
Learn more about Spanish verb conjugation