Assertion (A) : A jet aircraft moving at Mach Number equal to 1 travels faster at an altitude of 15 km than while moving at Mach Number equal to 1 near the sea level. Reason (R) : The velocity of sound depends on the temperature of the surrounding medium.

The reason (R) is correct: the speed of sound in an ideal gas depends only on temperature, not pressure or density, so sound speed follows the atmospheric temperature profile [1]. Numerical data show the speed of sound at typical cruise altitudes (~11–20 km) is about 295 m/s at −57°C, whereas at sea level (15°C) it is about 340 m/s [2]. Mach number is the ratio of true airspeed to local speed of sound, so Mach 1 corresponds to the local sound speed. Therefore an aircraft at Mach 1 at 15 km would have a lower true airspeed (~295 m/s) than Mach 1 at sea level (~340 m/s). Hence A is false and R is true [2].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > 20.2. Structure of The Earth's Atmosphere > p. 274
2. [2] https://www.nasa.gov/wp-content/uploads/2018/07/speed_of_sound_9-12.pdf