Fibre optic backbone refers to the main network infrastructure that uses fibre optic cables to transmit data over long distances. It serves as the central communication pathway for various systems and devices within a network. Among the given options, telecommunication is the most appropriate choice for fibre optic backbone.

Telecommunication:
- Fibre optic cables are extensively used in telecommunication networks to establish high-speed, reliable, and secure communication links.
- These cables consist of thin strands of glass or plastic that transmit data in the form of light pulses.
- The use of fibre optic cables in telecommunication allows for faster data transmission rates and larger bandwidth capacity compared to traditional copper cables.
- Telecommunication companies use fibre optic backbone networks to connect different locations, such as offices, data centers, and substations, ensuring seamless communication and data transfer.

Cable television:
- Although cable television networks also use fibre optic cables, they are not typically referred to as fibre optic backbones.
- Fibre optic cables are used in cable TV networks to deliver high-definition video content and improve signal quality.
- However, the backbone of a cable TV network usually refers to the main distribution hub that receives and distributes signals to different parts of a local area.

Delay lines:
- Delay lines are not related to fibre optic backbone networks.
- Delay lines are used in electronics to introduce a time delay in a signal, allowing for synchronization or signal processing purposes.
- They are commonly used in devices like oscilloscopes, radar systems, and communication systems but do not involve fibre optic cables.

Bus topology:
- Bus topology refers to a network configuration where all devices are connected to a central cable, known as a bus or backbone.
- While fibre optic cables can be used in bus topology networks, the term "fibre optic backbone" specifically refers to the main network infrastructure rather than the topology itself.

In conclusion, among the given options, telecommunication is known as fibre optic backbone as it extensively uses fibre optic cables for high-speed and reliable communication.