What are seen as "beads on string" structure when viewed under an electron microscope?

Nucleosomes

The correct answer is Option (4) → Nucleosomes     

In eukaryotes, there is a set of positively charged, basic proteins called histones. A protein acquires charge depending upon the abundance of amino acids residues with charged side chains. Histones are rich in the basic amino acid residues lysine and arginine. Both the amino acid residues carry positive charges in their side chains. Histones are organised to form a unit of eight molecules called histone octamer.

The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome . A typical nucleosome contains 200 bp of DNA helix.Nucleosomes constitute the repeating unit of a structure in nucleus called chromatin, threadlike stained (coloured) bodies seen in nucleus. The nucleosomes in chromatin are seen as ‘beads-on-string’ structure when viewed under electron microscope .

                          Nucleosome