Answer 1

You CAN use C with WASN know ! Yes, it is possible, even though it is not professionally used, as far as I know

Answer 2

A web app using javascript for front-end responds to the user without having to reload the page. This can't be achieved with C. So I think as far as webpages are concerned. Javascript is a faster responding language than C. Javascript should be used

Answer 3

You cannot use C in a static web site. Only interpreting languages can be used. You can use C-programs on the server side to create dynamic web sites. For this you have to use the CGI or FastCGI Interface. Both a techniques from last millenium :-). So I wouldn't recommend to invest time to learn CGI with C-programs.