Hint: To observe the nucleotide patterns in the smaller DNA, it requires special methods that can reach those small structures. A method of using a beam of light, with a far shorter wavelength than visible light, is used in this process.
X-ray crystallography can be used to see the nucleotide arrangement in DNA.
It is an experimental science that can determine the atomic and molecular structure of a crystal by a beam of incident X-ray. The beam passes through the crystalline structure and gets diffracted into many specific directions. By measuring the angles and intensities of the diffracted light one can determine the three-dimensional picture of the density of electrons inside the crystal. That's how the nucleotides are visualized.
As we all know, an electron microscope is a machine that uses a beam of accelerated electrons to investigate the ultrastructures of a variety of biological specimens including microorganisms, cells, biopsy samples, metals, etc. They have higher resolving power than light microscopes.
On the other hand, light or optical microscopes commonly use visible light and a system of lenses to magnify objects. These are simple and have weaker resolving power than an electron microscope or probe microscopes.
Ultracentrifuge is a centrifuge that is optimized for spinning a rotor at a very high speed generating acceleration up to 1000000g. They are used in the pelting of fine particulate fractions like cellular organisms and viruses.
So, the correct answer is option (a) "X-ray crystallography".
Note: Along with DNA, the structure of other biological molecules such as vitamins, drugs, proteins are revealed by the X-ray crystallography method. The crystals of these molecules are illuminated with the beam of X-ray to produce diffraction patterns of regularly spaced spots called reflections. The 2d pictures generated by this method is used to study the structure of such small molecules.