Final answer:
Option (b), Conflicts with Native Americans, including the defense of Spanish territories and coping with harsh frontiers, played a critical role in the failure of Spain's initial settlement attempts in the present-day United States.
Step-by-step explanation:
The failure of the first Spanish attempt to establish a settlement in present-day United States can be attributed to a combination of factors, but the option that stands out, given the context provided, is conflicts with Native Americans. Early settlers faced significant resistance from Indigenous peoples who were defending their lands and way of life.
Aditionally, the governance issues in Florida, disputes over borders, the closing of the Mississippi River by Spain, and the lack of control in Spanish Florida highlight the various challenges that Spain faced, which included defending its territory and managing relations with its neighbors. The harshness of the colonial frontiers often meant that settlers had to contend with challenging environmental conditions and resource scarcity, but political and territorial conflicts had a direct impact on the feasibility and success of Spanish settlements.