The correct answer is option 'C', Block Transfer.

Explanation:
DMA (Direct Memory Access) is a technique used in computer systems to transfer data between devices without involving the CPU. It improves the overall system performance by offloading the data transfer task from the CPU to a specialized DMA controller.

There are several DMA modes available, each with its own characteristics and performance levels. Among the given options, Block Transfer is the fastest DMA mode. Let's understand why:

1. Cycle Steal:
- Cycle Steal is a DMA mode where the DMA controller temporarily interrupts the CPU during each data transfer cycle to gain access to the system bus.
- In this mode, the DMA controller "steals" cycles from the CPU to transfer data between the device and the memory.
- Although it allows concurrent data transfer, it interrupts the CPU frequently, causing delays and reducing overall system performance.
- Hence, Cycle Steal is slower compared to other modes.

2. Hidden Mode:
- Hidden Mode is a DMA mode where the DMA controller operates in parallel with the CPU without interrupting it.
- It transfers data in the background, without affecting the CPU's execution.
- While Hidden Mode improves system performance by reducing interruptions, it may not be the fastest mode as it still shares the system bus with the CPU.

3. Block Transfer:
- Block Transfer is a DMA mode specifically designed for high-speed data transfer.
- In this mode, the DMA controller transfers a block of data from the device to memory or vice versa, without interrupting the CPU.
- The DMA controller gains control of the system bus and performs the transfer independently, allowing for efficient and uninterrupted data movement.
- As Block Transfer involves transferring a block of data at once, it reduces the overhead associated with frequent interruptions or small data transfers.
- This mode is particularly useful for devices that require fast data transfer rates, such as disk drives or network interfaces.
- Therefore, Block Transfer is considered the fastest DMA mode among the given options.

In summary, Block Transfer is the fastest DMA mode because it allows for uninterrupted high-speed data transfer between devices and memory.