As far as pure simplicity, it's impossible to narrow down this expression into fewer elements, as right now it is only composed of a numerator and a denominator, and the two elements don't share any factors in common, which means it can't be simplified like a normal fraction.
However, notice that there is a square root symbol in the denominator. Oftentimes, math teachers will ask that students do not leave answers in that form, so I'm assuming that is the case here and what "simplify" means in this context.
To effectively "remove" a square root, simply multiply it by itself. remember that if you multiply square root of x by square root of x, you are essentially squaring the square root, which just leaves you with x. Also keep in mind however, that you are simplifying the expression here, not changing the value. To deal with this, not only do you need to multiply the denominator by the square root of 3, but also the numerator. This way, you are effectively multiplying the current expression by 1 while getting rid of the square root in the denominator. After multiplying, the result is (2 root 3) / 3. Since the fraction can't be simplified down any farther, this is your final answer.