AlDS is caused by Human Immunodeficiency Virus (HIV). It has a single stranded RNA with a protein coating. After entry into the host cell, the single stranded RNA is changed into doublestranded DNA by a virally encoded reverse transcriptase. With the help of integrase, the viral DNA gets incorporated into the cellular DNA and starts replicating. It is characterized by the reduction in the numbers of CD4 -bearing helper T-cells to 20% or less of normal ones. T Iymphocytes play an important role in immune response and cell signaling. So, the correct answer is option A.