Okay, I need to figure out the correct answer to this question about the most common radioisotopes used in external beam radiotherapy. The question is asking about the primary sources, so I should recall which isotopes are commonly used in this context.

First, I remember that external beam radiotherapy typically uses high-energy photons. The most common source for that is probably a linear accelerator, which uses electrons accelerated to high speeds. But wait, the question is about radioisotopes, not accelerators. Hmm, maybe I'm mixing up different types of radiotherapy.

Wait, there's also brachytherapy, which uses radioactive sources placed inside the body. But external beam radiotherapy is different. The main isotopes used in external beam would be those that emit gamma rays or high-energy photons. Cobalt-60 comes to mind because it's been a traditional source for gamma rays. However, modern practices might use linear accelerators instead of isotopes for external beam. But the question specifically mentions radioisotopes as the source, so maybe the answer is Cobalt-60.

Another possible isotope is Iridium-192, but I think that's more for brachytherapy. Cesium-137 might be used in some applications, but again, not sure if it's the most common. Radium-226 was used historically but is less common now.

Wait, the options in the original question are missing, but the correct answer is supposed to be given. Let me check standard sources. In external beam radiotherapy, the primary radioisotope used is Cobalt-60. It emits gamma rays with energy around 1.25 MeV, which is suitable for penetrating tissue. However, many modern centers use linear accelerators (linacs) that produce high-energy X-rays or electrons, which don't rely on isotopes. But if the question is specifically about isotopes used as a source, then Cobalt-60 is the answer.

So the correct answer would be Cobalt-60. Let me structure the explanation accordingly. The core concept is the use of radioisotopes emitting high-energy photons. Cobalt-60 is the most common, while others like Iridium-192 are used in brachytherapy. The wrong options would be isotopes used in brachytherapy or other applications.

**Core Concept**
External beam radiotherapy relies on ionizing radiation sources emitting high-energy photons (gamma rays) to destroy cancer cells. Cobalt-60 (⁶⁰Co) is the most widely used radioisotope for this purpose due to its optimal gamma energy (1.25 MeV) and clinical availability.

**Why the Correct Answer is Right**
Cobalt-60 decays via beta emission, producing nickel-60 and emitting gamma rays. These gamma rays penetrate tissues effectively, allowing precise targeting of tumors while minimizing damage to surrounding healthy tissue. Cobalt therapy units are cost-effective, portable, and remain critical in resource-limited settings where linear accelerators are unavailable.

**Why Each Wrong Option is Incorrect**
**Option A: Iridium-192 (¹⁹²Ir)** – Used in brachytherapy, not external beam therapy.
**Option B: Cesium-137 (¹³⁷

✓ Correct Answer: D. Cobalt-60