To determine which of the options is rational, we need to evaluate each one.
A. \( \sqrt{3} + \sqrt{3} \) This simplifies to \( 2\sqrt{3} \), which is irrational because \( \sqrt{3} \) is irrational.
B. \( \frac{1}{\sqrt{3}} \) This is also irrational since \( \sqrt{3} \) is irrational.
C. \( \frac{\sqrt{3}}{1} \) This is simply \( \sqrt{3} \), which is irrational.
D. \( \sqrt{3} \times \sqrt{3} \) This simplifies to \( 3 \), which is a rational number.
Therefore, the only rational option is D. \( \sqrt{3} \times \sqrt{3} \).