Step 1: Understand the Speed of Light in Vacuum.
The speed of light in vacuum is denoted by \( c \) and is approximately:
\[
c = 3 \times 10^8 \, \text{m/s}.
\]
Step 2: Analyze Each Option.
Option 1: \( 8 \times 10^3 \, \text{m/s} \) — Incorrect, much smaller than \( c \).
Option 2: \( 8 \times 10^6 \, \text{m/s} \) — Incorrect, still smaller than \( c \).
Option 3: \( 3 \times 10^6 \, \text{m/s} \) — Incorrect, too small by two orders of magnitude.
Option 4: \( 3 \times 10^8 \, \text{m/s} \) — Correct, matches the speed of light.
Step 3: Final Answer.
\[
\boxed{3 \times 10^8 \, \text{m/s}}
\]