The correct answer is: B. the sum of the number of protons and neutrons present in the nucleus.
The mass number of a nucleus is the total number of protons and neutrons in the nucleus. The atomic number is the number of protons in the nucleus. The atomic weight is the average mass of one atom of an element, expressed in atomic mass units (amu).
The mass number is always greater than or equal to the atomic number, because the number of neutrons is always greater than or equal to zero. The mass number is always less than or equal to the atomic weight, because the atomic weight is affected by the mass of the electrons, which are not present in the nucleus.
Option A is incorrect because the mass number is always greater than or equal to the atomic number. Option C is incorrect because the mass number is always less than or equal to the atomic weight. Option D is incorrect because the mass number is not a fraction.