It seems like you are looking to modify the class of the li or another tag used in the menu when the page changes.
To achieve this, you can insert the following C# code within the master page's page_load event:
string currentPage = this.Page.GetType().Name.ToString();
switch (currentPage)
{
case "home_aspx":
liHome.Attributes.Add("class", "Active");
break;
case "support_aspx":
liSupport.Attributes.Add("class", "Active");
break;
case "logout_aspx":
liLogout.Attributes.Add("class", "Active");
break;
}
Make sure to replace "li" with the ID of the tag used in your menu.