how to bind dropdownlist which is inside repeater?

On your Repeater's ItemDatabound event use the following:

if (e.Item.ItemType == ListItemType.Item || 
         e.Item.ItemType == ListItemType.AlternatingItem)
{

    ((DropDownList)e.Item.FindControl("drp_CompPropAddress")).DataSource =(DataRowView) e.Item.DataItem;//Or any other datasource.
    ((DropDownList)e.Item.FindControl("drp_CompPropAddress")).DataBind();

}

Use the ItemDataBound event of the Repeater, like this:

protected void rep_UnAssignComps_ItemDataBound(object sender, RepeaterItemEventArgs e)
{
    DropDownList selectList = e.Item.FindControl("drp_CompPropAddress") as DropDownList;
    if (selectList != null)
    {
        selectList.DataSource = SomeDataSource(); //your datasource
        selectList.DataBind();

        //selectList.DataTextField = "SomeColumn";
        //selectList.DataValueField = "SomeID";
    }
}

Also remember to set the DataTextField and DataValueField properties, either in the markup or in the ItemDataBound event.