What name was taken by Cardinal Joseph Sarto when he became pope?

Pope Saint Pius X (born Giuseppe Melchiorre Sarto, (2 June 1835 – 20 August 1914) was head of the Catholic Church from August 1903 to his death in 1914. He was canonized in 1954. Pius X is known for vigorously opposing modernist interpretations of Catholic doctrine, promoting traditional devotional practices and orthodox theology. His most important reform was to order the codification of the first Code of Canon Law, which collected the laws of the Church into one volume for the first time. He was also considered a pastoral pope, in the sense of encouraging personal holiness, piety and a daily lifestyle reflecting deep Christian values. He was born in the town of Riese, which would later append "Pio X" to the town's name.

On 20 July 1903, Leo XIII died, and at the end of that month the conclave convened to elect his successor. It took five ballots to finalise the decision of the next Pope. After the fifth vote of the conclave was taken, and the count had Rampolla with 10 votes, Gotti with two votes, and Sarto with 50 votes. Thus, on 4 August 1903, Cardinal Joseph Sarto was elected to the pontificate.