Final answer:
4-H is the best fit for Nelson's background in agriculture as well as his education in engineering and science, due to the organization's focus on agricultural education and inclusion of STEM elements.
Step-by-step explanation:
Given Nelson's background in agriculture from growing up on a farm and his education in engineering and science, the youth organization that would be the best fit for his interests and skills is likely 4-H. This organization has a strong focus on agricultural education, hands-on learning, and leadership, which aligns well with Nelson's experience with cattle and sheep as well as his academic pursuits. Furthermore, 4-H programs often include STEM (Science, Technology, Engineering, and Mathematics) elements, which would allow Nelson to share his knowledge in engineering and science with youth members.