Explanation:

- Option 1: US$ 900 trillion

- This number is significantly exaggerated. The total global GDP doesn't even amount to US$ 900 trillion.

- Option 2: US$ 600 trillion

- Like option 1, this figure is unrealistically high. Even the largest economies have reserves in billions, not trillions.

- Option 3: US$ 600 billion

- ?? This is the correct figure. India's foreign exchange reserves crossed US$ 600 billion in June 2021 for the first time. This milestone reflected economic recovery and strong capital inflows.

- Option 4: US$ 900 billion

- This is higher than the actual reserves at that time. India had not reached this level by June 2021.