Under what condition can B-12.5 mix be substituted for SP-12.5 TL-D or E?

The correct choice indicates that B-12.5 mix can be substituted for SP-12.5 TL-D or E at no extra cost, but only within a limit of 500 tons. This condition is essential because it allows for flexibility in materials used while maintaining project cost efficiency. The limitation of 500 tons ensures that the substitution does not significantly impact the project's integrity or design specifications.

This approach underscores the importance of adhering to specified guidelines and standards in paving mixtures while also allowing contractors to manage resources effectively without incurring additional costs. It ensures that the quality and performance characteristics of the pavement are preserved, as both mixes share similar properties that make the substitution viable within stated constraints.