The reason thunder is heard long after the flash of lightning is; option [D] The speed of light is much greater than the speed of sound. Lightning, which causes thunder, emits light that travels at a speed of approximately 300,000,000 meters per second, much faster than the speed of sound at about 343 meters per second in air. Consequently, the flash of lightning is seen almost instantly, while the sound of thunder takes longer to reach observers due to its slower propagation speed.

This delay allows us to estimate the distance of a lightning strike by counting the seconds between seeing the lightning and hearing the thunder (since sound travels approximately 1 kilometer in 3 seconds). The phenomenon underscores the fundamental difference in speed between light and sound waves, influencing how we perceive and interpret distant events like thunderstorms. Therefore, the correct answer explaining the delay is [D] The speed of light is much greater than the speed of sound.