What is meant by the beads on a string model of chromatin?

What is meant by the beads on a string model of chromatin?
 
  A) The beads are the histones, and the string is the transcriptionally active DNA loops.
  B) The beads are the nucleosomes, and the string is the linker DNA.
  C) The beads are molecules of DNA polymerase that attach to the DNA string.
  D) The beads are ribosomes, and the string is the mRNA that has been transcribed from active chromatin.
  E) The beads are the heterochromatic regions that are tightly compacted, and the strings are euchromatic regions that are being actively transcribed.