Final answer:
The appropriate present tense form is 'está'.
Step-by-step explanation:
The appropriate present tense form of either ser or estar to fill in the blank in the sentence 'La clase de la Sra. Álvarez, ______ muy difícil.' is está. The verb 'estar' is used to indicate temporary states or conditions, and in this sentence, it is appropriate to use 'está' to describe the difficulty level of the class, which can vary over time. Example: 'La clase está muy difícil esta semana' (The class is very difficult this week).
Learn more about Present tense form of 'ser' and 'estar'