To solve this problem, we need to identify which cells HIV targets first when it causes AIDS. Let's analyze the options:$\newline \bullet$Option 1: B-lymphocytes$\newline$ - B-lymphocytes are responsible for producing antibodies, but they are not the primary target of HIV.$\newline \bullet$Option 2: Leucocytes$\newline$ - Leucocytes, or white blood cells, include various types of immune cells, but HIV specifically targets a subset of these.$\newline \bullet$Option 3: Helper T-lymphocytes$\newline$ - Helper T-lymphocytes, also known as CD4+ T cells, are the primary target of HIV.$\newline$ - HIV binds to the CD4 receptor on these cells, leading to their destruction.$\newline$ - The loss of helper T-lymphocytes impairs the immune system, leading to AIDS.$\newline \bullet$Option 4: Thrombocytes$\newline$ - Thrombocytes, or platelets, are involved in blood clotting and are not targeted by HIV.$\newline$Based on this analysis, the correct answer is Option 3: Helper T-lymphocytes.