how can i make this code more optimized

Easy enough. You're only ever making one item visible depending on the case option, so just set the visibility as follows:

pnlDate.Visible = (selVal == "date"); pnlSubject.Visible = (selVal == "subject");pnlofficer.Visible = (selVal == "officer");pnlCIA.Visible = false;pnlMedia.Visible = false;pnlStatus.Visible = (selVal == "status");    

This is better than setting everything to visible = false; and then only showing the item you need as this contains it all into just 6 lines of code for the actual visibility setting.

Another way:

// set everything to false   Dictionary<string, type> d = new Dictionary<string, type>(){    {"date", pnlDate},    {"subject", plnSubject},    {"officer", plnOfficer},    {"status", plnStatus}};d[selVal].Visible = true;

protected void ddlFilterResultBy_SelectedIndexChanged(object sender, EventArgs e)            {                 string selVal = ddlFilterResultBy.SelectedValue.ToString().ToLower();                        pnlDate.Visible = false;                        pnlSubject.Visible = false;                        pnlofficer.Visible = false;                        pnlCIA.Visible = false;                        pnlMedia.Visible = false;                        pnlStatus.Visible = false;                switch (selVal)                {                    case "date":                        pnlDate.Visible = true;                                            break;                    case "subject":                        pnlSubject.Visible = true;                        break;                    case "officer":                        pnlofficer.Visible = true;                        break;                     case "status":                        pnlStatus.Visible = true;                        break;                  }            }