**Core Concept**
Oxygen concentrators are medical devices that use pressure swing adsorption (PSA) technology to concentrate oxygen from room air, which contains approximately 21% oxygen. This technology separates oxygen from nitrogen and other gases, allowing for the delivery of a high concentration of oxygen to patients.

**Why the Correct Answer is Right**
The correct answer is **C. 90%**. Oxygen concentrators work by using a series of pressure vessels and valves to alternate between two modes: adsorption and desorption. During the adsorption mode, the air is passed through a bed of activated carbon, which removes nitrogen and other gases, leaving a high concentration of oxygen. This oxygen is then released to the patient. The concentration of oxygen supplied by oxygen concentrators is typically around 90%, which is sufficient for most patients requiring supplemental oxygen.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because oxygen concentrators do not supply 100% oxygen, as this would require a different technology, such as liquid oxygen or oxygen generators.

**Option B:** This option is incorrect because the concentration of oxygen supplied by oxygen concentrators is not typically around 50%, as this would be closer to the concentration of oxygen in room air.

**Option D:** This option is incorrect because the concentration of oxygen supplied by oxygen concentrators is not typically around 60%, as this would be closer to the concentration of oxygen in room air.

**Clinical Pearl / High-Yield Fact**
It's essential to note that oxygen concentrators are not suitable for patients requiring high concentrations of oxygen, such as those with severe respiratory failure or carbon monoxide poisoning. In these cases, a different oxygen source, such as a medical oxygen tank, may be necessary.

**Correct Answer: C. 90%**