Final answer:
The hip bone's iliac crest is the preferred and most practical site for obtaining a red bone marrow sample, despite being a painful procedure, due to its easy accessibility and isolation from vital organs.
Step-by-step explanation:
The best location for obtaining a red bone marrow sample from a patient is the hip bone (C), particularly the region near the iliac crest of the pelvic bones. This site is traditionally preferred for its accessibility, being close to the body's surface and relatively isolated from most vital organs, which makes it a safer choice for bone marrow extractions, despite the procedure being quite painful. It is important to note that while the femur, frontal bone, and head of humerus contain red bone marrow, the hip bone's iliac crest is the most commonly used and practical site for bone marrow biopsies.