Answer:
You must select at least 21 strings of length 6.
Step-by-step explanation:
There are 2⁴ = 16 strings of each type.
Since there are 4 types of strings, then in order to have 6 strings of the same type you need to have at least 21 strings selected. The worst case will that there are 5 strings for each of the other 3 types.