To find the distance between the points \( P(x, y) \) and \( Q(-x, -y) \), we use the distance formula:
\[
d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}
\]
Here, \( (x_1, y_1) = (x, y) \) and \( (x_2, y_2) = (-x, -y) \). Substituting these values into the distance formula:
\[
d = \sqrt{((-x) - x)^2 + ((-y) - y)^2}
\]
\[
d = \sqrt{(-2x)^2 + (-2y)^2} = \sqrt{4x^2 + 4y^2} = 2 \sqrt{x^2 + y^2}
\]
Step 2: Conclusion.
The distance between the points \( P(x, y) \) and \( Q(-x, -y) \) is \( 2 \sqrt{x^2 + y^2} \). So, the correct answer is (C).