$("#ddlSource option[value='1']").attr("selected", "selected");
Here, ddlSource is the ID of the HTML select [drop down list] control. The meaning of the above statement is, find the control which has the ID "ddlSource" and in it find the option which has give value and select it. "1", I am using for example. You can pass any value.
If you are using the server side controls, and want to populate the value in JQuery, then use below statement.
$("#<%=ddlSource.ClientID %> option[value='1']").attr("selected", "selected");
For best practice Please try to use <select runat="server" ID="ddlSource" ></select> instead of <asp:DropDownList /> to avoid the invalid callback error messages. I know, the immediate question in your mind, how to get the value using <select> control in C# code. Use below code for it.
string selectedValue = Request.Form[ddlSource.UniqueID]; //UniqueID is because Form is expecting name instead of ID.
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