Why was Fort St. Louis a threat to Spain? A) It was the center of French and American Indian operations against Spain in the
Americas.
B) It was a French fort within territory that Spain believed was part of New Spain.
C) It was a very effective military outpost for France.
D) It was the capital of a new French colony near New Spain.

B) It was a French fort within the territory that Spain believed was part of New Spain.

Explanation:

Fort St. Louis was a fort established by French Explorer La Salle and his group in Texas in 1685 and was initially believed to be near the Mississippi River.

However, in 1686, when the Spanish settlers were aware of the Fort St. Louis, they believed it could serve as a threat to their control over New Spain which includes the territory of Texas and surroundings.

Hence, the right answer to the question is "It was a French fort within the territory that Spain believed was part of New Spain."