First, it is important to understand the terms used in the problem.
The Arithmetic Mean: (this is the mean we are used to seeing)

(n= number of variables, in this case n=3)
To Find the Geometric Mean of n #'s:
![m_(geometric) = \sqrt[n]{a*b}](https://img.qammunity.org/2018/formulas/mathematics/high-school/zvqjvggelajj7zuy5oepkulkquvajzbred.png)
(n= number of variables, in this case n=2)
So, using this information we can find the geometric mean of 6,7 by plugging them into the equation as "a" and "b". n=2 (when n=2 a square root can be used)
![m_(geometric) = \sqrt[n]{a*b}](https://img.qammunity.org/2018/formulas/mathematics/high-school/zvqjvggelajj7zuy5oepkulkquvajzbred.png)


So the answer in this case is A.