Dropdownlistbox contains four options “Add”,”Sub”,”Mul” and “Div”. Bydefault selected option is “Add”. arithmetic operation on Text box value and result display in lable

You need to set up an event handler for the ListBox so when the selection changes, your code uses the result to decide which operation to perform. There are plenty of samples around that show the basic functionality of listboxes.

Use switch statement in selectedindex_cheanged event of dropdownlist