Final answer:
Starbucks' engagement with suppliers and purchasing a coffee farm improves its supply chain sustainability by ensuring continuous access to resources and quality control.
Step-by-step explanation:
Developing long-term relationships with suppliers and purchasing a coffee farm in Costa Rica would most notably improve supply chain sustainability for Starbucks. These actions contribute to a secure, stable, and responsible supply chain by ensuring continual access to coffee, reducing risk related to supply disruption, and allowing more control over quality and sourcing practices. By specializing in their operations, as explained in the provided reference information, Starbucks would effectively leverage comparative advantage, allowing for improved efficiency and profitability within its supply chain framework.