Is the motif "Lord of the Flies" microcosm, extended metaphor, controlling image, structure, or progression?For my Lit class, we are doing a motif...

I would argue that the Lord of the Flies
is more microcosm.  By choosing boys, and by isolating them on an island,
Golding is creating an insular society within the real society.  This society the boys
create is a microcosm of our own society.  The little society made of boys has all of
the elements of ours.  It has leaders and followers, a crude system of government,
cruelty, greed and corruption.  By using boys and depicting a society that descends to
anarchy, Golding was sending the message that we are never very far away from chaos. 
This is a common theme in literature.  When you take away the trappings of society,
people return to their animal instincts.  But all of the elements are already there in
our won society.