We got promising results by using radioactive isotopes for the treatment of:

The correct option is C cancer
We got promising results by using radioactive isotopes for the treatment of cancer.
Co-50 $\left(\gamma \right)$ is used to check cancer growth.
Co-60 $\left(\gamma \right)$ + Au-198 $\left(\gamma \right)$ is used to check cancer and tumor treatment.
Other applications of artificial radioactive isotopes in medicines are as given below.
Na - 24 $\left(\gamma \right)$ is used to detect tumors and blood clots. P-32 is used in the treatment of leukemia.