The answer to this question is definitely 'A'.Let me explain why. HIV infects T cells that carry CD4 antigen on their surface i.e CD4 T cells.In addition HIV can also affect macrophages that bear CD4 on their surface. The first step in the HIV infection is the binding of envelope glycoprotein of HIV to the CD4 receptor on the T cells/macrophages.How ever this interaction alone is not sufficient for entry and infection of the virus. It needs a second receptor i.e a co-receptor.HIV has two coreceptors- CCR5 CXCR4. Therefore HIV must bind to CD4 receptor and one of the above coreceptor to enter into the cell. Any damage to these coreceptors will prevent HIV from entering the cell, thus leading to resistance to HIV infection. A well known example is a mutation of CCR5 which is called delta 32 mutation. Those persons with this mutation is resistant to HIV infection because HIV virus cannot bind to this mutated CCR5 and therefore cannot enter the cell.