The correct answer is option 'C': Third generation.

Explanation:
The development of notable features like keyboards, monitors, and GUI (Graphical User Interface) occurred during the third generation of computers. Let's discuss each of these features in detail:

1. Keyboards:
- Keyboards were introduced as an input device during the third generation of computers.
- They replaced punched cards and paper tape as the primary means of entering data and commands into the computer.
- Keyboards allowed users to input alphanumeric characters, numbers, and special symbols directly into the computer.

2. Monitors:
- Monitors, also known as display screens or visual display units (VDUs), became a common output device during the third generation of computers.
- Monitors provided a visual representation of data and information processed by the computer.
- Users could view text, images, and graphical elements on the monitor screen, making it easier to interact with the computer.

3. GUI (Graphical User Interface):
- The development of the Graphical User Interface (GUI) was a significant advancement during the third generation of computers.
- GUI introduced a visual and intuitive way for users to interact with computers.
- Instead of relying solely on text-based commands, users could navigate through menus, icons, and graphical elements using a mouse or other pointing devices.
- GUI made computers more accessible to a wider range of users and contributed to the popularization of personal computers.

In summary, the notable features like keyboards, monitors, and GUI were developed during the third generation of computers. These advancements revolutionized the way users interacted with computers, making them more user-friendly and accessible.