The correct answer is B. Sit back on the chair.
Step-by-step explanation:
Nowadays, it is common many employees had to work long hours in the same position; this can lead to multiple health issues, for example, sitting all day can cause back pain, and muscle tension. In these cases, the first recommendation is to sit in an adequate position that involves sitting back on the chair rather than on the edge, reclining the back, keeping the shoulders relaxed but without leaning forward and keep the feet on the floor.
Besides this, it is recommended to stop working for a couple of minutes to walk or stretch the muscles. Thus, the guideline the doctor would advise Mary to follow is "Sit back on the chair" because this is part of sitting properly which is the most important to avoid back pain and other health consequences.