The size of a 99Mo/99Tc generator is expressed as the total activity of:

The correct choice reflects the standard practice in nuclear medicine regarding the characterization of a 99Mo/99Tc generator. The size or capacity of a generator is commonly expressed in terms of the total activity of 99Mo present on the column. This is because the parent isotope, 99Mo, is what undergoes decay to produce the daughter isotope, 99mTc. The amount of 99Mo determines how much 99mTc can ultimately be generated.

When evaluating the generator's capabilities, it's essential to consider the initial activity of 99Mo because it sets the limit for how much 99mTc can be eluted over time. The specific decay rate of 99Mo and its half-life also play crucial roles, as they impact the timing and quantity of 99mTc available for use. Hence, the total activity of 99Mo serves as a crucial indicator of the generator's potential output of 99mTc, rather than merely the amounts present on the column or the amounts that can be eluted at a given time without considering decay corrections.