hi, the answer is a) but i don't quite understand restrictive vs nonrestrictive modifiers. could someone please explain to me what the difference is? thanks

The author Herman Melville and the poet Walt Whitman are icons of American literature, greatly beloved by generations past and present.
a) The author Herman Melville and the poet Walt Whitman are icons
b) Herman Melville the author and Walt Whitman the poet are icons
c) The author named Herman Melville and the poet named Walt Whitman are great icons
d) The author, Herman Melville, and the poet, Walt Whitman, are icons
e) Herman Melville, the author, and Walt Whitman, the poet, had been icons