Under Article XVIII, Section 25, after the expiration in 1991 of the Military Bases Agreement with the United States, foreign military bases, troops or facilities shall not be allowed in the Philippines except under:

The correct answer is B. A treaty duly concurred in by the Senate and, when Congress so requires, ratified by a majority of votes cast in a national referendum

Section 25 requires a treaty concurred in by the Senate, ratified by the people in a referendum when Congress requires it, and recognised as a treaty by the other contracting state. The Senate rejected a new bases treaty in September 1991.