A.
This is taking place// at a time when //the world is witnessing// an unprecedented movement// of people from rural areas //to cities and other centers.
B.
This is taking place at a time when// the world is witnessing an unprecedented movement// of people from rural areas// to cities and other centers.
C.
This is taking place at a time when// the world is witnessing an unprecedented movement of people from rural areas to cities// and other centers.
D.
This is taking place at a time// when the world is witnessing an unprecedented movement of people// from rural areas to cities// and other centers.