Any person or persons acting in concert, who together own . . . . . . . . or more of the open interest in index derivatives are required to disclose the same to the clearing corporation.

The correct answer is: 5%.

Any person or persons acting in concert, who together own 5% or more of the open interest in index derivatives are required to disclose the same to the clearing corporation. This is to prevent market manipulation and to ensure that the market is fair and orderly.

Option A: 35% is incorrect because it is not the required disclosure threshold.

Option B: 15% is incorrect because it is not the required disclosure threshold.

Option C: 5% is the correct answer because it is the required disclosure threshold.

Option D: 1% is incorrect because it is not the required disclosure threshold.