Final answer:
Methotrexate is an immunosuppressant agent and a DMARD used in the treatment of autoimmune diseases.
Step-by-step explanation:
Methotrexate is an immunosuppressant agent and a DMARD (Disease-Modifying Antirheumatic Drug) used in the treatment of various autoimmune diseases, such as rheumatoid arthritis.
It works by inhibiting the activity of certain immune cells and reducing inflammation.
Therefore, option a. Methotrexate is the correct answer.