Manganese, which has the atomic number 25, has an atomic mass of about 54.94. To find the number of neutrons, we subtract the atomic number from the atomic mass. Since the atomic mass is not a whole number, we can round it to 55 for the purposes of this calculation.
Calculation: Number of Neutrons = Atomic Mass – Atomic Number
Number of Neutrons = 55 – 25 = 30
Therefore, the correct answer is b) 30. This means that the most common isotope of manganese has 30 neutrons in its nucleus.