The correct answer is "a. Are found."
The sentence is referring to the presence of certain rare species of plants in the Amazon. Since the subject "many important but rare species of plants" is plural, the verb "find" should also be plural. The present tense of "to be" (are) is required to match the plural subject, and the past participle of "find" (found) should be used to create the present perfect tense. Therefore, the correct sentence is "Many important but rare species of plants are found in the Amazon."