Block transfer and Vectored interrupts enable the highest I/O bandwidth.

Block Transfer:
Block transfer is a DMA transfer mode where data is transferred in blocks rather than individual bytes. In this mode, the DMA controller transfers a block of data from the I/O device to memory or vice versa without the need for CPU intervention after the initial setup. This allows for efficient and fast data transfer as multiple bytes can be transferred in a single operation.

Vectored Interrupts:
Vectored interrupts are a type of interrupt handling mechanism where the interrupting device provides additional information, such as the address of the interrupt service routine (ISR), along with the interrupt request. This allows the CPU to directly jump to the specific ISR without the need for searching through a vector table. Vectored interrupts reduce the overhead associated with interrupt handling and enable faster response times.

Reasoning:
- Transparent DMA is a transfer mode where the DMA controller takes control of the system bus and transfers data directly between the I/O device and memory. However, it does not involve interrupt handling mechanisms. Therefore, it does not provide the highest I/O bandwidth as the CPU cannot perform other tasks during the transfer.
- Cycle-stealing is a transfer mode where the DMA controller temporarily halts the CPU's operation to transfer a small number of data bytes. It is not as efficient as block transfer for high-bandwidth data transfer.
- Polling interrupts involve the CPU actively checking for interrupt requests from devices. This can be time-consuming and inefficient for high-bandwidth data transfer as the CPU spends a significant amount of time checking for interrupts instead of performing other tasks.

Summary:
- Block transfer mode allows for efficient transfer of data in blocks, reducing the overhead associated with transferring individual bytes.
- Vectored interrupts provide faster interrupt handling by directly jumping to the specific ISR, reducing the search time in a vector table.
- Combining block transfer and vectored interrupts enables the highest I/O bandwidth as it allows for efficient data transfer and faster interrupt handling.