Select the correct phrase to complete the alien ambassador's rather unusual peace treaty.

The Galactic Council has agreed that Earth will be spared from immediate vaporization, ____ your world leaders supply us with a weekly tribute of chocolate chip cookies.

The correct answer is on condition that.

"On condition that" is a formal conditional conjunction meaning "only if," which perfectly fits the stipulation of a treaty. "In case" expresses a precaution, "lest" means "for fear that," and "even though" introduces a factual concession rather than a condition.