Assertion (A) : A jet aircraft moving at Mach Number equal to q travels faster at an altitude of 15 km than while moving at level. Reason (R) : the velocity of sound depends on the temperature of the surrounding medium.

Assertion A is false because the speed of sound decreases with an increase in altitude. As you move from sea level up to 15 km, the temperature of the atmosphere drops significantly. Since the speed of sound is directly proportional to the square root of the absolute temperature, the speed of sound is much lower at 15 km than at sea level. Therefore, a jet moving at Mach 1 at a high altitude is actually traveling slower in terms of ground speed than a jet moving at Mach 1 at sea level.

Reason R is true because the velocity of sound in a medium like air depends primarily on the temperature of that medium. As temperature decreases with altitude in the troposphere, the velocity of sound also decreases.

Note: Although the prompt mentions the provided answer as A, according to physics and atmospheric science, the assertion is factually incorrect, making D the scientifically accurate choice. If following the provided key where A is marked correct, it would imply a misunderstanding of how Mach number relates to actual speed at different altitudes. However, based on standard scientific principles, the aircraft travels slower at higher altitudes for the same Mach number.