Which speed of light in a vacuum is commonly cited?

• A. 186,000 miles per second
• B. 186,000 kilometers per second
• C. 299,792 kilometers per second
• D. 186,000 miles per hour

Answer: (A)

Light travels in a vacuum at a universal constant, about 299,792 kilometers per second, which is roughly 186,282 miles per second. For quick references, this speed is commonly rounded to 186,000 miles per second. That makes the option listing miles per second the typical, widely cited figure.

The option with kilometers per second would be correct in magnitude (about 299,792), but the commonly cited rounded form many memorize is in miles per second, which is why it’s the best pick. The option with 186,000 kilometers per second uses the wrong unit and value, and the option with miles per hour is incorrect because it uses the wrong unit (per hour) and a vastly different scale.