Step 1: Concept
The velocity of electromagnetic waves in vacuum is a fundamental concept derived from Maxwell's equations. It relates to the permeability ($\mu_0$) and permittivity ($\epsilon_0$) of free space.
Step 2: Meaning
$\mu_0$ represents the magnetic permeability of free space, while $\epsilon_0$ denotes the electric permittivity of free space. These constants are intrinsic properties of the vacuum and are used to describe how electromagnetic fields behave in a vacuum.
Step 3: Analysis
The velocity ($v$) of an electromagnetic wave in a medium is given by the equation:
\[v = \frac{1}{\sqrt{\mu_0 \epsilon_0}}\]
This formula comes from the relationship between the speed of light and the properties of the medium. In vacuum, this simplifies to the velocity of electromagnetic waves.
Option A) $1 / \sqrt{\mu_0 \epsilon_0}$ matches exactly with the derived equation for the velocity of electromagnetic waves in a vacuum.
Options B), C), and D) do not match the correct form:
Option B) $\sqrt{\mu_0 \epsilon_0}$ would give an incorrect velocity.
Option C) $\mu_0 \epsilon_0$ is the product of permeability and permittivity, which does not represent a velocity.
Option D) $1 / (\mu_0 \epsilon_0)$ would also yield an incorrect result.
Step 4: Conclusion
The correct expression for the velocity of electromagnetic waves in vacuum is derived from the inverse square root of the product of $\mu_0$ and $\epsilon_0$.
Final Answer: (A)