Low-level buffer filling refers to the process of filling a buffer at a low-level, closer to the hardware. In this context, the low-level refers to the hardware level of a computer system, where the operations are performed directly on the hardware devices.

Among the given options, the correct answer is option 'C', which is the DMA (Direct Memory Access) controller. Let's understand why.

DMA Controller:
- A DMA controller is a hardware device that allows for direct data transfer between peripherals and memory without the intervention of the CPU.
- It is responsible for managing data transfers between the memory and peripherals such as disk drives, network cards, sound cards, etc.
- The DMA controller fills the buffer directly from the peripheral, bypassing the CPU, which significantly reduces the CPU's involvement in the data transfer process.
- This low-level buffer filling performed by the DMA controller helps in improving the overall system performance, as it offloads the CPU from the burden of handling data transfer tasks.

Other options:
a) Output:
- Output refers to the process of sending data from a computer system to an external device or display.
- However, low-level buffer filling is not typically associated with output operations.
- While a buffer may be used for storing output data temporarily, the filling of the output buffer is usually done by the CPU or higher-level software.
- Therefore, output does not have low-level buffer filling.

b) Peripheral:
- A peripheral is an external device connected to a computer system, such as a printer, scanner, keyboard, etc.
- While data transfer between the CPU and peripherals may involve buffers, the filling of these buffers is again typically performed by the CPU or higher-level software.
- Hence, peripherals do not have low-level buffer filling.

d) Input:
- Input refers to the process of receiving data into a computer system from an external source, such as a keyboard, mouse, or network.
- Similar to output and peripherals, the filling of input buffers is usually done by the CPU or higher-level software.
- Thus, input does not have low-level buffer filling.

In conclusion, the DMA controller is the only option among the given choices that involves low-level buffer filling.