The correct answer is (a) 235.
Uranium-235 is an isotope of uranium with 92 protons and 143 neutrons. The number of protons in the nucleus of an atom determines its chemical element, while the number of neutrons determines its isotope. Uranium-235 is the most common isotope of uranium, accounting for about 0.7% of all uranium atoms. It is a fissile isotope, meaning that it can be split by neutrons to release energy. This process is called nuclear fission, and it is the basis of nuclear power generation.
Option (b), 92, is the number of protons in the nucleus of uranium-235. This is the same for all isotopes of uranium.
Option (c), 143, is the number of neutrons in the nucleus of uranium-235. This is the number that distinguishes uranium-235 from other isotopes of uranium.
Option (d), 51, is the number of protons in the nucleus of tin. This is the number that distinguishes tin from other elements.