What causes the activity of the fixer to diminish over time?

The correct answer regarding what causes the activity of the fixer to diminish over time is that the fixer solution accumulates soluble silver salts. In radiographic testing, the fixer solution plays a crucial role in removing unexposed silver halide crystals from the film. As the fixer works, it captures these silver salts, which leads to a change in the chemical composition of the fixer.

Over time, the accumulation of these dissolved silver salts in the fixer solution reduces its effectiveness, as the chemical agents may become saturated and unable to effectively remove additional unexposed silver halide. This decrease in the kinetic activity of the fixer leads to slower processing times and potentially poorer quality images if the film remains in the solution for an extended period.

Additionally, while factors such as exposure to air and temperature can influence the stability and effectiveness of chemical solutions, they do not directly impact the core chemical activity the way the accumulation of silver salts does. Concentration changes can occur, but the primary reason for the decrease in activity specifically relates to the buildup of these materials which directly affects the solution's ability to perform its intended function.