Final answer:
To determine the number of protons in an isotope with a mass number of 129 and 74 neutrons, one subtracts the number of neutrons from the mass number, resulting in 55 protons. So the correct option is B.
Step-by-step explanation:
The mass number of an isotope is the sum of its protons and neutrons. Since the mass number is given as 129 and the number of neutrons is 74, we can find the number of protons by subtracting the number of neutrons from the mass number. Therefore, the number of protons is calculated by:
number of protons = mass number - number of neutrons
number of protons = 129 - 74 = 55
Hence, the isotope has 55 protons in its nucleus, which corresponds to option B.