This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
Immunostimulant agent among the following is : 
Select and explain. 
A. Pirenzepine, B. Levamisol, C. Albendazole, D. Methotrexate
B: Levamisol
Answer: B: Levamisol 
Explanation: None