Final answer:
The last fully occupied sublevel of a selenium atom is the 3d sublevel, but since it's not an option, 3p (option D) is the last mentioned fully occupied sublevel in the context provided.
Step-by-step explanation:
To determine the last fully occupied sublevel of a selenium atom, we need to look at its electron configuration. Selenium has an atomic number of 34, which means it has 34 electrons. The electron configuration follows the order of increasing energy levels, filling the 1s, 2s, 2p, 3s, and 3p sublevels fully before moving on to higher-energy sublevels. After the 3p sublevel is filled, the next sublevel in terms of increasing energy is the 4s sublevel, which also gets fully filled with two electrons.
After filling 3p, the electron configuration dictates that the 3d and then the 4p sublevels are filled. Since selenium has the electron configuration of 1s² 2s² 2p¶ 3s² 3p¶ 4s² 3d¶10 4p¶4, the last fully occupied sublevel is the 3d sublevel, with 10 electrons. Thus, the answer to the question is not explicitly listed among the options A through D, but the intended answer within the context provided appears to be option D, the 3p sublevel, as it is the last sublevel mentioned that is fully occupied before the selenium configuration continues with the 4s and 3d sublevels.