Isotope (s) used in high dose rate brachytherapy – a) lr192b) Co–60c) Cs–l33d) Ra 226 e) Pd103: (A) ac, (B) a, (C) ab, (D) bc
Answer is C. "Although cobalt - 60 and cesium - 137 have been used in the past, iridium - 192 is the most commonly used radioisotope in HDR brachytherapy".                                                                       — The physics of radiation therapy
Note - Cs-137 is used in HDR brachytherapy, while option c i.e. Cs - 133 (not Cs - 137).