Which philosopher holds that "happiness is desirable and the only thing desirable as an end; all other things being only desirable as a means to that end? "?

The correct answer is John Stuart Mill. 
He was a famous British philosopher who lived during the 19th century. He was a major proponent of utilitarianism, which is an idea that everything we do is supposed to help the society around us. According to him, even happiness is a utilitarian though, meant to help the world.