Final answer:
The most prominent characteristic of the Mexican Institutional Revolutionary Party (PRI) was corruption.
Step-by-step explanation:
The most prominent characteristic of the Mexican Institutional Revolutionary Party (PRI) was corruption. The PRI effectively ran a one-party state where all presidents and nearly all members of the legislature were members of the party. While there were regular elections and an expansion of voting, the PRI maintained a system of patronage and was notorious for its corrupt practices.