After the September 11 attacks, NATO invoked Article 5 of its treaty which stated that allied nations must support

Aa nation under attack.
Ca nation attacking another nation.
Ba nation in war.
Dany nation in the treaty for any reason.

Verified Answer :

Article 5 of the NATO treaty states that allied nations must support "d. any nation in the treaty for any reason," since it says that a nation in the treaty under attack means that all the nations in the treaty are under attack.