Final answer:
An atom of 131I (iodine) contains 78 neutrons, which is calculated by subtracting the atomic number of iodine (53) from its mass number (131).
Step-by-step explanation:
An atom of 131I contains 78 neutrons. The atomic number of iodine (I) is 53, which is the number of protons in its nucleus. To find the number of neutrons, we subtract the atomic number from the mass number (131 - 53 = 78). Therefore, option b (78 neutrons) is the correct answer.