The statement that best identifies why Texans Julián Castro, Henry Cisneros, and Ron Kirk served on modern presidential cabinets is:
They had political experience as former mayors of large Texas cities.
All three individuals held mayoral positions in large Texas cities prior to their appointments to presidential cabinets. Julián Castro was the mayor of San Antonio, Henry Cisneros was also the mayor of San Antonio, and Ron Kirk was the mayor of Dallas.