The correct option is C Radio-isotopes
Tracer elements are radioisotopes which are used to trace the path of molecules in metabolic pathways. Thus elucidating the metabolic pathway. For example an important experiment proving that DNA carries genetic information, was based on tracer techniques. In 1952 Alfred D. Hershey and Martha Chase used radioactive phosphorus (32P) and radioactive sulfur (35S) tracers to show that when the bacterial virus (bacteriophage)T2 infects its host cell, Escherichia coli, it is the phosphorus-containing DNA of the viral particle, not the sulfur-containing protein of the viral coat, that enters the host cell and furnishes the genetic information for viral replication.