how to used obout grid inside dropdownlist

DropDownList ddlSTANDINGFtr = (DropDownList)GridView1.FindControl("ddlSTANDINGFtr");
 

string com = "SELECT ID,SLNO FROM TBL WHERE CODE='" + ..... + "' AND STATUS= 0";
DataTable dt = Lib.FnFetchData(com);
ddlSTANDINGFtr.DataSource = dt;
ddlSTANDINGFtr.DataBind();
ddlSTANDINGFtr.DataTextField = "SLNO";
ddlSTANDINGFtr.DataValueField = "SLNO";
ddlSTANDINGFtr.DataBind();
ddlSTANDINGFtr.Items.Insert(0, new ListItem("--Select--", "0"));

take a look at these articles. they would explain how to use dropdownlist in a gridview.
Use DropDownList in ASP.NET GridView Control
Using RadioButtonList and DropDownList in GridView Row