Hi ivinraj25,
I have cross checked your code and its working properly.
Refer below sample code and modify the code according to your need.
MasterPage.master
<%@ Master Language="C#" AutoEventWireup="true" CodeFile="MasterPage.master.cs" Inherits="MasterPage" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<script type="text/javascript" src="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" />
<asp:ContentPlaceHolder ID="head" runat="server">
</asp:ContentPlaceHolder>
</head>
<body>
<form id="form1" runat="server">
<div>
<ul class="nav nav-pills nav-main" id="mainMenu">
<li class="<%=(System.IO.Path.GetFileName(Request.PhysicalPath).ToString().ToUpper() == "DEFAULT.ASPX" ? "active" : " ")%>">
<a href="Default.aspx">Home</a></li>
<li class="<%=(System.IO.Path.GetFileName(Request.PhysicalPath).ToString().ToUpper() == "ABOUTUS.ASPX" ? "active" : " ")%>">
<a href="AboutUs.aspx">About Us</a></li>
<li class="<%=(System.IO.Path.GetFileName(Request.PhysicalPath).ToString().ToUpper() == "ACADEMICS.ASPX" ? "active" : " ")%>">
<a href="Academics.aspx">Academics</a></li>
<li class="<%=(System.IO.Path.GetFileName(Request.PhysicalPath).ToString().ToUpper() == "COURSES.ASPX" ? "active" : " ")%>">
<a href="Courses.aspx">Courses</a></li>
</ul>
<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
</asp:ContentPlaceHolder>
</div>
</form>
</body>
</html>
Default.aspx
<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true"
CodeFile="Default.aspx.cs" Inherits="_Default" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
<h2>
Default Page.
</h2>
</asp:Content>
AboutUs.aspx
<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true"
CodeFile="AboutUs.aspx.cs" Inherits="AboutUs" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
<h2>
AboutUs Page.
</h2>
</asp:Content>
Academics.aspx
<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true"
CodeFile="Academics.aspx.cs" Inherits="Academics" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
<h2>
Academics Page.
</h2>
</asp:Content>
Course.aspx
<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true"
CodeFile="Courses.aspx.cs" Inherits="Courses" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
<h2>
Courses Page.
</h2>
</asp:Content>
ScreenShot
