Final answer:
The thymus is the correct answer as it is a primary lymphoid organ where T cells mature. The spleen, appendix, and tonsils are secondary lymphoid organs, which play a role in filtering lymph and blood and initiating immune responses.
Step-by-step explanation:
The question asks which option listed is considered a primary lymphoid organ. The primary lymphoid organs are where lymphocytes, like B and T cells, mature and learn to distinguish between the body's own cells and foreign cells (pathogens). In this case, option C) thymus is the correct answer because the thymus is a primary lymphoid organ where T cells mature. The other options listed, such as the spleen, appendix, and tonsils, are all considered secondary lymphoid organs, which do not produce or mature lymphocytes but instead are involved in filtering lymph and blood, and in initiating adaptive immune responses once lymphocytes have matured elsewhere.