Explanation:

Cobalt therapy or cobalt 60 therapy is the medical use of gamma rays from the radio isotope cobalt −60 to treat conditions such as cancer. Thus, Cobalt −60 is the radio isotope useful in treating the blood cancer. Cobalt therapy is the medical use of gamma rays from the radioisotope cobalt-60 to treat conditions such as cancer. Beginning in the 1950s, cobalt-60 was widely used in external beam radiotherapy (teletherapy) machines, which produced a beam of gamma rays which was directed into the patient's body to kill tumor tissue. Hence option 2nd is correct.