First of all, chlorine only gives one acid such as HCl, and that is HCl itself. It seems like you want to have a talk about acidity, which is perfectly fine, but refuse to consider anything but HCl.
Second, I'll give you what you want. HCl is a strong acid because chlorine is a very electronegative element (second only to fluorine), which means that the dissociation of HCl gives a very stable chloride anion. Other effects to consider are bond strength (which is rather small) and hydration energy (which measures the stabilization of the ions). There's more to it, but I'm afraid it will be too difficult for you.
Either way, if you really want to know why HCl is a very strong acid, you will have to compare it to other acids to understand all possible influences.