Explanation:

- Option 1: India and Japan

- These two nations have a strong defense relationship and conduct various exercises, but "Keen Sword" is not one of them.

- Option 2: India and USA

- India and the USA engage in numerous joint military exercises, such as "Yudh Abhyas," but not "Keen Sword."

- Option 3: USA and Japan

- "Keen Sword" is a regular exercise involving armed forces from the USA and Japan to enhance interoperability and combat readiness.

- This exercise reflects their close military alliance and strategic cooperation.

- Option 4: Japan and Taiwan

- Japan and Taiwan do maintain informal defense relationships, mainly due to political sensitivities with China, but they do not conduct the "Keen Sword" exercise together.