Answer (D)
Option A: Yes, it can be invoked by Javascript. Invoking a servlet is just like making a call to web page.
Option B: Yes, it can be invoked by a Applet. Infact anyone capable of calling a webpage can call a servlet. Because servlet is processed on server and client only gets HTML form (page) of it, so client does not need to be aware ideally about the servlet, other than the input params (if any) to pass.
Option C: Yes it is executed on server side and client only gets HTML form.
Option D:This is wrong. Only one instance is created and that instance is used to process client request(s). For each client request, a new thread is created which uses same instance of servlet.