I am calling function from .asmx.cs file by ajax web method. Now, I am getting output undefined value in ajax success.

Here is my code,

<input type="button" id="btnGetStock" value="View" class="button-89" />

$("#btnGetStock").click(function (e) {
    var dataToSend = {
        labelno: document.getElementById("<%=txt_labelno.ClientID%>").value
    };
    $.ajax({
        type: "POST",
        url: "FillGridMethod.asmx/QuotationList",
        data: dataToSend,
        dataType: "json",
        success: function (data) {
            alert(JSON.stringify(data.ItemType));
            $("#lbl_type").val(data.ItemType);
        }
    });
});

Here is the .asmx file

[WebMethod(enableSession: true)]
public void QuotationList(string labelno)
{
    if (HttpContext.Current.Session["BranchId"].ToString() == null)
    {
        Server.Transfer("Index.aspx");
        return;
    }
    var quotation = new List<QuotationModel>();
    string constr = cn.ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        qryFillGrid = " select ItemType from tbl_StockRunning stock " + System.Environment.NewLine;        
        var cmd = new SqlCommand(qryFillGrid, con);
        con.Open();
        var dr = cmd.ExecuteReader();
        while (dr.Read())
        {
            var quotationModel = new QuotationModel
            {
                ItemType = dr[0].ToString().Trim()
            };
            quotation.Add(quotationModel);
        }
    }
    var js = new JavaScriptSerializer();
    Context.Response.Write(js.Serialize(quotation));
}

Now, when I am doing alert of alert(JSON.stringify(data)); then I am getting value in correct Like [{“ItemType”:”GOLD”}] format. But when I am doing alert(JSON.stringify(data.ItemType)); then it getting undefined