Question

The most suitable instrument for measuring the size of an atom is:

Answer 1

In a covalent molecule, the distance between consecutive atoms can be used to estimate the size of one atom. A chlorine atom's covalent radius is half the distance between the nuclei of the atoms in a $C{l_2}$ molecule, for example.

Complete answer:
Only an electron microscope has resolution of $0.5 - 0.6{\text{ }}Angstrom$ which is less than the size of an atom.
An electron microscope is a microscope that illuminates with a beam of accelerated electrons. Electron microscopes offer a higher resolving power than light microscopes and may expose the structure of smaller objects since the wavelength of an electron is 100,000 times shorter than that of visible light photons. Most light microscopes are restricted by diffraction to roughly $200nm$ resolution and usable magnifications below 2000, but a scanning transmission electron microscope has achieved more than $50pm$ resolution in annular dark-field imaging mode and magnifications up to about 10,000,000.
Shaped magnetic fields are used in electron microscopes to create electron optical lens systems that are similar to the glass lenses used in optical light microscopes.
Microorganisms, cells, big molecules, biopsy samples, metals, and crystals are among the biological and inorganic specimens that electron microscopes are used to analyse the ultrastructure of. Electron microscopes are frequently used in industry for quality control and failure analysis. Modern electron microscopes create electron micrographs by capturing images with sophisticated digital cameras and frame grabbers.

Note:
The transmission electron microscope has a number of drawbacks, one of which is the requirement for extremely thin specimen sections, often less than $100nanometers$ . It's technically difficult to make these tiny sections for biological and materials specimens. A concentrated ion beam can be utilised to create semiconductor thin sections.