Reducing the time you spend near a radiation source lowers the dose you receive. The amount of exposure is directly related to how long you are exposed, so spending only the minimum time needed to complete a task is the straightforward way to apply time management for protection. The option described matches this idea exactly: it says to cut the time near the source to the minimum necessary to finish the task, which directly reduces dose. By contrast, extending time near the source, waiting for a shield to fail, or increasing exposure to test detectors all raise or unnecessarily expose you, which goes against safe practice. This choice best demonstrates minimizing exposure through time control.

Protons define the element, and isotopes share the same number of protons. Iodine has 53 protons (its atomic number), so every isotope of iodine has 53 protons. The notation iodine-131 means the total number of nucleons (protons plus neutrons) is 131, so neutrons are 131 minus 53, which is 78. Therefore, the number of protons is 53. The other numbers correspond to neutrons or the total nucleons, not to the proton count.

Biological harm is what we’re trying to compare when we talk about radiation dose, so the quantity used is dose equivalent. This takes the energy actually deposited in tissue (the absorbed dose) and adjusts it for how damaging different types of radiation can be. The unit that represents this adjusted, biologically relevant dose historically is the rem (Roentgen Equivalent Man). It effectively multiplies the absorbed dose by a radiation weighting factor to reflect greater or lesser biological impact, giving a single value that relates to potential harm to a person. One rem equals 0.01 sievert, with sievert being the modern SI unit. Other measures shown—Roentgen quantifies exposure in air, not dose to tissue; Gray measures energy deposited (absorbed dose) without regard to biological effectiveness; RAD is an older absorbed-dose unit—do not directly express how harmful the radiation might be. Hence, REM is the best choice for a dose measure based on biological effect.

When water is exposed to a neutron field, the abundant oxygen-16 in the water undergoes a quick neutron-induced reaction to become nitrogen-16. This happens mainly through an (n,p) reaction: oxygen-16 absorbs a neutron and emits a proton, turning into nitrogen-16. Nitrogen-16 then decays with a short half-life of about 7 seconds, emitting high-energy gamma rays as it returns to oxygen-16. That rapid production and decay makes nitrogen-16 the classic activation product of reactor coolant water. Ar-41 comes from activation of argon in the surrounding air, not from the water itself. Tritium (H-3) can be formed if deuterium in water is activated, but deuterium is only a tiny fraction of natural water, so the immediate, characteristic activation product of water in a reactor context is nitrogen-16.