Very similar to how you've already done the other If statements:
<li class="nav-item <%# IfTrue(IsDocumentOnSelectedPath(),"active") %>">
<a class="nav-link <%# IfTrue(IsDocumentOnSelectedPath(),"active") %>" href="<%# GetDocumentUrl() %>"><%# Eval("MenuItemName") %>
<%# IfTrue(IsDocumentOnSelectedPath(), "<span class=\"sr-only\">(current)</span>") %>
</a>
<cms:SubLevelPlaceHolder runat="server" Id="plcSub" />
</li>