Which of the following is a radioisotope used to date rock formations older than 50,000 years old?

The radioisotope used to date rock formations 50 000 years ago is Uranium. This radioactive uranium isotope having a mass number of 235 and its symbol is U, its atomic number is 92, and the mass number is 238, comprising 0.715 percent of natural uranium. When bombarded with neutrons it undergoes fission with the release of energy.