R spectroscopy can be used to identify which are present in a compound.

IR spectroscopy can be used to identify chemical structures are present in compounds.

Explanation:

Infrared spectroscopy is a technique in organic chemistry that can be use use to identify chemical structures present in compounds because it is base on the ability of different functional groups to adsorb infrared light.

This work by shinning the infrared lights into the organic compounds to be identified, some of the frequencies of the infrared lights are adsorbed by the compounds and its identify groups of atoms and molecules in the compound.

a:

by the combination of an unlimited number of elements

i think that the answer is c