To keep single pane open always

@using Syncfusion.EJ2.Navigations;

@(Html.EJS().Accordion("defaultAccordion")
    .Clicked("clicked")
    .Expanding("beforeExpand")
    .Items(new List<AccordionAccordionItem> {
        new AccordionAccordionItem { Header = "ASP.NET", Expanded = true, Content = "Microsoft ASP.NET is a set of technologies in the Microsoft .NET Framework for building Web applications and XML Web services." },
        new AccordionAccordionItem { Header = "ASP.NET MVC", Content = "The Model-View-Controller (MVC) architectural pattern separates an application into three main components: the model, the view, and the controller." },
        new AccordionAccordionItem { Header = "JavaScript", Content = "JavaScript (JS) is an interpreted computer programming language.It was originally implemented as part of web browsers so that client-side scripts could interact with the user, control the browser, communicate asynchronously, and alter the document content that was displayed." }
    })
    .Render()
)

<script type="text/javascript">
    var clickEle;
    function clicked(e) {
        clickEle = e.originalEvent.target;
    }
    function beforeExpand(e) {
        var expandCount = this.element.querySelectorAll('.e-selected').length;
        var ele = this.element.querySelectorAll('.e-selected')[0];
        if (ele) {
            ele = ele.firstChild;
        }
        if (expandCount === 1 && ele === clickEle) {
            e.cancel = true;
        }
    }
</script>

public ActionResult Index()
{
    return View();
}