The correct answer is B. Serial.

A serial cable is a type of cable that can be used to connect devices that use serial communication, which is a method of sending data one bit at a time over a single wire. Serial cables are often used for console access to network devices, such as routers, switches, firewalls, or servers, when other methods of connection, such as SSH (Secure Shell), are not available or not working.

A DB9 connector is a type of connector that has nine pins and is commonly used for serial cables. DB stands for D-subminiature, which is a family of connectors that have a D-shaped metal shield and different numbers of pins. DB9 connectors are also known as DE-9 connectors, as they belong to the E size category of D-subminiature connectors. DB9 connectors can support various serial standards, such as RS-232, RS-422, or RS-485.

A parallel cable is a type of cable that can be used to connect devices that use parallel communication, which is a method of sending data multiple bits at a time over multiple wires. Parallel cables are often used for connecting printers or scanners to computers, but they are not commonly used for network devices or serial connections.

An optical cable is a type of cable that can be used to transmit data using light signals over fiber-optic strands. Optical cables can offer high-speed, high-bandwidth, and low-interference data transmission over long distances. Optical cables are often used for network communication, such as Ethernet, or audio/video transmission, such as HDMI or TOSLINK.

A USB cable is a type of cable that can be used to connect devices that use the Universal Serial Bus (USB) standard, which is a protocol that allows data and power transmission between devices. USB cables can support various types of devices, such as keyboards, mice, cameras, flash drives, or chargers. USB cables have different versions and connectors, such as USB-A, USB-B, USB-C, or Micro-USB.

Therefore, the only cable that uses a DB9 connector is a serial cable. For more information about cables and connectors, you can refer to the following web search results: [What Is a Serial Cable?], [What Is a Parallel Cable?], [What Is an Optical Cable?], [What Is a USB Cable?].