Final answer:
The U.S. war in Afghanistan from 2001 to 2021 is best described by the foreign policy approach of interventionism, which involves active international engagement and can include military action.
Step-by-step explanation:
The foreign policy approach that describes the U.S. war in Afghanistan from 2001–2021 is b. Interventionism. This approach is characterized by a nation actively engaging in international affairs, which may include the use of military force. The aim often involves promoting the country's values or interests, responding to threats, or maintaining global stability.
Isolationism would mean the U.S. avoiding international engagement, which is contrary to its actions in Afghanistan. Realism and Pacifism are other approaches to foreign policy, but neither accurately captures the active and military-based nature of the U.S. involvement in Afghanistan. Instead, interventionism reflects the decision to deploy troops and resources in response to the September 11 attacks and to attempt to dismantle terrorist networks and establish stable governance in the country.