What practice, explicitly banned by the Decemviri, was legalized again five years after their second term in office?

The correct answer is intermarriage between patricians and plebs, which reflects a significant social development in ancient Roman society. After the creation of the Twelve Tables by the Decemviri, there was a strict prohibition against intermarriage between the patrician and plebeian classes. This restriction highlighted the deep social divide between these two groups in Rome.

However, five years after the Decemviri's second term, the social and political pressures from the plebeians, combined with the gradual shifts in power dynamics, led to a more egalitarian approach that included the legality of intermarriage. This change was crucial in facilitating greater social integration and weakening the rigid class distinctions that had characterized the early Republic.

The lifting of this ban reflects broader themes of reform and the ongoing struggles of the plebeians to gain more rights and recognition within Roman society. It underscored the gradual movement towards a more inclusive society where social mobility became increasingly possible through familial connections, which was pivotal as the Republic evolved.