Answer:
"Arizona" is the adjective in the sentence.
Step-by-step explanation:
Adjectives are those words that add to, modify or describe a noun in a more specified way. They give more meaning and description to the nouns or pronouns they are associated to. In the following sentence-
"In North America, I have photographed cactus plants from Mexico to Canada, and I think Arizona cacti are the most beautiful."
the writer is talking of the various cactus plants that he/she had photographed. And among those, he finds the Arizona cacti the most beautiful. By naming the cacti as a specific being, 'Arizona' is used as an adjective in this case as it gives the description of the particular cactus that the narrator finds most beautiful.