Final answer:
President Obama most strongly emphasizes the quality of community in the given sentences, highlighting the importance of working together as one nation to achieve educational and infrastructural goals. The correct answer is option A.
Step-by-step explanation:
President Obama emphasizes the quality of community most strongly in the sentences provided. He stresses the idea that working together as a nation, unified in effort, is essential to address the challenges of training adequate math and science teachers, building infrastructure, and creating jobs. His reference to doing these things together 'as one nation, and one people' clearly underscores the importance of collaboration and collective action in achieving national goals.