Which best identifies the underlined clause?

most of these animals stay under water because they cannot tolerate exposure to the air for extended periods of time. - is underlined

a. noun clause
b. independent clause
c. adverb clause
d. adjective clause

This is not an independent clause: it is dependent on the main phrase here, which is "Most of these animals stay under water".

The underlined clause tells us something about why they must stay under water; and sentences that tell us about the why are called the "adverb clauses" - the correct answer.