What are those structures that appear as beads-on strings in the chromosomes when viewed under an electron microscope?
A) Genes
B) Nucleotides
C) Nucleosomes
D) Base pairs

Hint:Chromosomes are thread-like structures found in the nucleus of plants and animals. It is made up of protein and molecules of DNA(deoxyribose nucleic acid). It contains genetic material which is passed from parent to offspring.

Complete answer:
Packaging of DNA helix:
Taken the distance between two consecutive base pairs as 0.34 nm $(0.34 \times 10^{-9} m)$, if the length of DNA double helix in a typical mammalian cell is calculated ( by taking product of the total number of bp with distance between two consecutive bp, that is, $(6.6 x\times 109 bp \times 0.34 \times 10^{-9} m/bp)$, it comes out to be approximately 2.2 metres.

Histones are a special group of proteins found in the nuclei of eukaryotic cells responsible for DNA packaging leading to chromatin formation. This is important because without histones DNA would be extremely long in relation to the cell.

Eukaryotic chromatin structure consists of repeating units, known as nucleosomes, which resemble beads on a string connected by sequences of linker DNA.

A nucleosome is formed of DNA sections 200 bp (base pairs) wrapped around a core octamer of eight histones.

Histones contain a large proportion of the positively charged (basic) amino acids, lysine and arginine in their structure.

DNA is negatively charged due to the phosphate groups on its backbone. The result of these opposite charges is strong attraction and therefore high binding affinity between histones and DNA.

The nucleosome chain gives a bead on string' appearance under electron microscope. A cylindrical coil is formed by a beaded cylinder or solenoid having 6 nucleosomes per turn.

Thus the correct answer is option (B) i.e. Nucleosome

Note:The nucleosomal organisation provides a chromatin fibre approximately 10 nm in thickness. This string further coils up into a solenoid of 30 nm diameter and then into a filament of 200 nm diameter. Further supercoiling gives rise to 700 nm filament and ultimately metaphase chromosome of 1400 nm.