What is the speed of light in a vacuum?

The question asks for the speed of light in a vacuum.

The speed of light in a vacuum, denoted by $c$, is a fundamental physical constant. Its approximate value is $299,792,458 \, \text{m/s}$, which is commonly rounded to $3 \times 10^8 \, \text{m/s}$.

Let's examine the given options:

• a) $3 \times 10^8 \, \text{m/s}$: This is the correct and widely accepted value for the speed of light in a vacuum.
• b) $3 \times 10^6 \, \text{m/s}$: This value is incorrect; it is two orders of magnitude smaller.
• c) $3 \times 10^4 \, \text{m/s}$: This value is incorrect; it is four orders of magnitude smaller.
• d) $3 \times 10^2 \, \text{m/s}$: This value is incorrect; it is six orders of magnitude smaller.

The correct speed of light in a vacuum is $3 \times 10^8 \, \text{m/s}$.

The final answer is $\boxed{\text{A}}$