Final answer:
Type II concrete is suitable for use in drain structures that require moderate sulfate resistance, designed to withstand sulfate exposure, unlike Type I or Type III.
Step-by-step explanation:
The best type of concrete mix to use in a drain structure that requires moderate sulfate resistance is Type II concrete. This category of concrete is specifically designed to provide protection against moderate sulfate attack.
Type V concrete is used when high sulfate resistance is required, whereas Type I is the general-purpose mix that is not intended for environments with sulfate exposure.
Type III concrete is known for its high early strength, but it is not the ideal choice for sulfate resistance. Therefore, in a scenario where moderate sulfate resistance is needed for a drain structure, Type II concrete would suffice.