Correct answer: (d) Steel
Explanation
- A
Vacuum
Vacuum cannot carry sound; there is no elastic medium. Velocity in vacuum is not highest — it is zero for sound. This option is the opposite of a sound path.
- B
Air
Air carries sound slowly (about 330–340 m/s at room conditions), the lowest of the three material media listed. Air is not the key.
- C
Water
Water carries sound faster than air (about 1400–1500 m/s) but slower than steel. Water is the middle medium, not the maximum.
- D
Steel
(d) Official key: Sound has the highest velocity in steel among vacuum, air, water and steel. Solids transmit longitudinal sound faster than liquids and gases. Steel is the official fastest medium here.
Summary. Official key is (d). Sound is fastest in steel among the options. Vacuum carries no sound; air is slowest of the materials; water is intermediate. Keep steel. The official letter is (d).