Speed of light in vacuum.

**The reasoning** The speed of light in a vacuum is a **fundamental constant of nature**, symbolized as *c*. It's approximately **300,000,000 meters per second** or **3 × 10⁸ m/s**. This value is fixed and appears everywhere in physics—from Einstein's E = mc² to calculations about electromagnetic waves, satellites, and even how quickly you see lightning before hearing thunder. To remember: Light travels about **300,000 kilometers per second** (since 1 km = 1000 m, that's 3 × 10⁵ km/s = 3 × 10⁸ m/s). It's the fastest speed possible in our universe. **Why the wrong options tempt you** The powers of 10 confuse students! - **Option A (10⁵)** is actually the speed of light in *km/s*, not m/s—a unit mix-up trap. - **Option B (10⁶)** and **Option D (10¹⁰)** are random large numbers that "feel" fast but aren't grounded in reality. Examiners know you might panic and pick anything that looks big. **Quick takeaway** Lock this in: **c = 3 × 10⁸ m/s**—eight zeros, universal speed limit, never changes. When you see "speed of light," see "10⁸" instantly! 🚀