Final answer:
Among the provided options, a dose of 4 mrem and 'No dose' are considered safe gonadal radiation doses. However, 'No dose' is the safest, and 4 mrem is a very low dose that is not typically known to cause significant health effects.
Step-by-step explanation:
The safe gonadal radiation dose for a person is considered to be a dose that minimizes the risk of long-term health issues such as cancer or genetic changes. In the options provided, 4 mrem (option 1) is closest to the average annual background radiation exposure of < 100 mrem and is thus considered to be a safe level. No radiation dose (option 2) is obviously the safest, but 4 mrem is within the range of very low doses that are not typically known to cause significant health effects. Doses of 5 rem (option 3) and 0.5 rem (option 4) are significantly higher and are associated with an increased risk of harmful biological effects. Therefore, out of the given options, 4 mrem (option 1) and 'No dose' (option 2) are considered safe, with 'No dose' being the safest, as smaller doses have lower associated risks according to the linear hypothesis. However, for the purposes of practicality in a world where low levels of environmental radiation are normal, 4 mrem is considered a 'safe' dose.