The radical sign (more commonly known as the square root sign) actually means the POSITIVE square root of a given number. This positive square root is often referred to as the principal square root. Anytime we see the radical sign on the GRE, we want the principal (positive) square root.
Also, from Wikipedia:
"Every non-negative real number a has a unique non-negative square root, called the principal square root, which is denoted by √, where √ is called radical sign. For example, the principal square root of 9 is 3, denoted √9 = 3 , because 3^2 = 3×3 = 9 and 3 is non-negative."
You can also check out this blog post on square roots for further reading! :)