- B. Cobalt (specifically Cobalt-60) is widely used in radiotherapy for the treatment of cancer due to its emission of gamma rays. This matches with application 3. - C. Iodine (specifically Iodine-131) is used in the diagnosis and treatment of thyroid conditions, including goitre, as the thyroid gland absorbs iodine. This matches with application 1. - D. Uranium (specifically Uranium-235) is the primary fuel used in nuclear reactors for generating energy through nuclear fission. Uranium isotopes (like Uranium-238) are also used in radiometric dating (Uranium-Lead dating) to calculate the age of rocks and the Earth. Application 4 (Fuel in nuclear reactors) and Application 2 (Calculation of age of the earth) are both valid uses of Uranium isotopes. - A. Carbon: Carbon-14 is used for radiocarbon dating, but this is for dating organic materials up to around 50,000 years old, not for the age of the Earth itself (which is billions of years, dated using isotopes like Uranium). Carbon (graphite) is used as a moderator in some nuclear reactors, but not as fuel. Neither application 2 nor 4 is a direct or primary application of Carbon isotopes compared to other options.