The blood vessels collapse when:

Explanation

The correct answer is A. External pressure applied becomes greater than the systolic pressure. This causes the blood vessel to collapse because the external pressure overcomes the internal pressure within the vessel, leading to its closure. Option B is incorrect because when the pressures are equal, the vessel does not collapse but blood flow may be impeded. Option C is incorrect as external pressure less than systolic pressure allows normal blood flow without vessel collapse. Option D is incorrect because zero external pressure does not interfere with the vessel's function, allowing normal circulation.