Pope Urban VIII was instrumental in the construction of Saint Peter's Basilica primarily through his decision to appoint Gianlorenzo Bernini as the principal architect responsible for its design and rebuilding. This role allowed Bernini to express the grandeur and spiritual aspirations of the Catholic Church during the Baroque period. Under Urban VIII’s patronage, Bernini became the key figure in transforming the basilica into a monumental work that symbolized the power of the papacy and the Catholic faith, culminating in the grand facade and the impressive interior spaces that we appreciate today.
Urban VIII's choice to involve Bernini signified a significant moment in art history, as it aligned the papacy with the innovative and deeply expressive artistic movements of the time. This collaboration not only reinforced the aesthetic values of the Baroque era but also contributed to the overall identity of the Catholic Church during a period marked by the Counter-Reformation, where visual art became a critical tool for communicating religious messages. The other options, such as designing the entire basilica, commissioning the Vatican walls, or designing papal apartments, do not accurately reflect Urban VIII's influence on Saint Peter's Basilica in the same profound way as his appointment of Bernini.