The correct choice is (c) Ogg
To explain I would say: The Ogg container format with the Theora video codec and the Vorbis audio codec is supported in desktop/mobile Gecko (Firefox), Chrome, and Opera, and support for the format can be added to Safari (but not on iOS) by installing an add-on. The format is not supported in Internet Explorer in any way. The most widely used format for HTMl video i.e MP4 format is supported in IE.