Final answer:
The new sentence about the saguaro cactus growing 15 to 50 feet tall best fits after sentence 3 of June's report, aligning with the information about the cactus's growth pattern.
Step-by-step explanation:
June's addition to her research report is a detail about the growth characteristics of a saguaro cactus. Given the context of the original paragraph, her sentence regarding height effectively follows the introductory claim that a saguaro grows slowly. It contrasts with the gradual aspect of its growth by emphasizing the ultimate height the cactus can reach. Therefore, inserting the new sentence after sentence 3 flows logically as it expands upon the cactus's growth pattern.