Which one of the following is least likely to be occupationally exposed to mercury vapors?

Farm workers are least likely to be occupationally exposed to mercury vapors compared to the other professions listed. The primary activities associated with farming typically do not involve the use of materials or processes that release mercury vapors.

In contrast, lab technicians often work with various chemicals, which can include mercury, especially in research or analytical labs where mercury is used in some analytical instruments or experiments. Machine operators in certain industrial settings might also be exposed to mercury, particularly if they work with machinery that uses or interacts with products containing mercury. Plumbers can encounter mercury through the use of certain types of piping and compounds that may have been used in older plumbing systems, as well as when working with certain medical waste that contains mercury.

Given these contexts, farm workers are generally engaged in agricultural activities that do not involve mercury, making them the least likely to be exposed to mercury vapors.