Answer:
338 different options we will have to choose from.
Explanation:
Number of base drinks = 13
Numbers of flavors = 26
If we can select 1 base and 1 flavor.
For every base drink we have 26 flavors.
Then for each 13 base drinks we will have 26 flavors.
So, number of options if we have If we select 1 base and 1 flavor:

338 different options we will have to choose from.