The problem involves a square where the diagonal is given as 6 units, and we need to find the length of the side $s$ of the square.

Since the diagonal of a square divides it into two 45°-45°-90° right triangles, we can apply the properties of such a triangle. In a 45°-45°-90° triangle, the relationship between the side lengths is:

$\text{Diagonal} = s\sqrt{2}$

Here, the diagonal is given as 6 units. Therefore, we can set up the equation:

$s\sqrt{2} = 6$

To solve for $s$, divide both sides by $\sqrt{2}$:

$s = \frac{6}{\sqrt{2}}$

Rationalize the denominator:

$s = \frac{6}{\sqrt{2}} \times \frac{\sqrt{2}}{\sqrt{2}} = \frac{6\sqrt{2}}{2} = 3\sqrt{2}$

Thus, the length of side $s$ is $3\sqrt{2}$.

The correct answer is C. $3\sqrt{2}$.

Do you want further details or have any questions? Here are five related questions:

1. What is the formula for the diagonal of a square in terms of its side length?
2. How do you rationalize a denominator in a fraction like $\frac{1}{\sqrt{2}}$?
3. What are the properties of a 45°-45°-90° triangle?
4. How can you find the area of the square from the given diagonal?
5. How would this problem change if the square's diagonal was a different length?

Tip: When working with special triangles, it's helpful to remember the side length ratios to quickly solve problems involving diagonals or hypotenuses.