Which type of due process requires legislation to be reasonable and serve a purpose?

Substantive due process is the correct choice as it pertains to the substance of legislation and whether a law is justifiable. This type of due process ensures that laws are not only fair in their application but also serve a legitimate governmental purpose. It scrutinizes the content of laws to ensure that they do not infringe on fundamental rights or are unjust, regardless of how they are enacted or enforced.

In essence, substantive due process evaluates whether laws that potentially infringe on constitutional rights are reasonable and serve a significant public interest, striking a balance between individual freedoms and government powers. This concept is often applied in cases involving fundamental rights, such as those related to privacy, marriage, and family.