The correct symbol for an isotope of iodine with 53 protons and 78 neutrons is B 131I.
To explain further:
- The number of protons determines the element (53 protons indicates it is iodine).
- The atomic mass of the isotope is the sum of protons and neutrons. Here, 53 protons + 78 neutrons = 131.
Therefore, the correct notation for the isotope would be written as \(^{131}I\), which is typically just shown as 131I in shorthand notation.