In which sentence does the subject correctly agree with the verb? . the saguaro cactus, native to southern arizona and northern mexico, have been known to live as long as two hundred years.. the branches of a saguaro, also known as arms, are covered with white flowers in the spring and red fruit in the summer.. when a saguaro dies, either the woody ribs or the hollow “saguaro boots” is used for construction and water storage.. temperature and elevation, both important environmental factors, affects the growth and health of a saguaro.

Only the sentence: "The branches of a saguaro, also known as arms, are covered with white flowers in the spring and red fruit in the summer." Follows correct subject-verb agreement.

In the other sentences, the following corrections should have been made:

"The saguaro cactus, native to southern Arizona and northern Mexico, have been known to live as long as two hundred years."
= "have" --> "has"

"When a saguaro dies, either the woody ribs or the hollow “saguaro boots” is used for construction and water storage."
= "is" --> "are"

"Temperature and elevation, both important environmental factors, affects the growth and health of a saguaro."
= "affects" --> "affect"