A radioactive nuclide has 53 protons and 78 neutrons. Which is the symbol of this nuclide?

The mass number is the total number of protons and neutrons: 53 + 78 = 131. The atomic number is 53, which identifies the element as iodine (I). Therefore, the nuclide is written with 131 as the mass number and 53 as the atomic number next to I, matching choice B.

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