What is the block signal system called in which the use of each block is governed by an automatic block signal, cab signal or both?

The correct answer refers to the Automatic Block Signal System (ABS), which is designed to enhance train safety and track utilization by automatically managing the use of track blocks. In this system, both automatic block signals and cab signals work in tandem to determine when a block can be occupied by a train. The automatic block signals control train movement by displaying signals based on the occupancy of the section of track ahead, allowing for continuous automatic management of train spacing and speed.

This system not only promotes safety by reducing the risk of collisions and derailments but also ensures that train operations can proceed without the need for manual intervention. The automatic nature of the block signal system enhances efficiency in train dispatching and allows for more fluid train operations.

The other options describe different types of signaling systems that do not utilize the automation aspects of the block signal system in the same way. The manual block signal system relies on human operators to control the use of blocks, interlocking signal systems focus on managing movements at junctions or crossings through a mechanical or electrical lock, and control block signals are typically used in specific contexts that may not involve automatic operations. Each of these alternatives lacks the comprehensive automatic functionality present in the ABS, which is vital to the answer.