When did the tectonic history of the Appalachian Mountains begin? A. 800-700 M. Y.B. P.
B. 500-400 M. Y.B. P.
C. 400-300 M. Y.B. P.
D. 300-260 M. Y.B. P.

480 million years ago

The Appalachians first formed roughly 480 million years ago during the Ordovician Period, and once reached elevations similar to those of the Alps and the Rocky Mountains before they were eroded.

Explanation:

so b is your answer