Tell me which mechanism enables the server controls to maintain their state ?

The mechanism which enables controls to maintain their state that is called View State.

Which property can we use to cause a postback to the server when you change the checked state of a CheckBox in the browser?

To select or clear it, We need to set the AutoPostBackproperty to True state to have a CheckBoxcontrol submit back to the server.

<asp:CheckBox ID="CheckBox1" runat="server" AutoPostBack="True"/>
Describe How the asp.net runtime keeps track of control state between postbacks?

The ASP.net run time uses the hidden field called __VIEWSTATE to store the values for the controls. This hidden field is sent with each postback to the server, where it's unpacked and then used to repopulate the controls in the page with their previous values.

Tell me the three different ways to add existing files to an ASP.NET web site in VWD ?

The first way open the Solution Explorer and then Right-click your project,and choose Add Existing Item and then browse for the item(s) you want to add.

The Second alternate way, you can drag and drop files from Windows Explorer or from your desktop directly into a VWD project.

Otherwise you could put the files directly in the project's folder using Windows Explorer.

How are the XHTML and HTML related?

XHTML is a stricter version of HTML that is rewritten with XML rules. In html sometimes we can use the code without any structure but in XHTML we should follow the rules and structure strictly, so that we can get the results that we are expecting. For example in XHTML elements must be properly nested and Attribute names must be in lower case and documents must have one root element and etc.