To disable extended selection mode, you can press the Esc key. When extended selection mode is enabled, you can use the arrow keys to select a block of text or data in a document or spreadsheet. The selected block is highlighted, and you can perform actions such as copying, cutting, or deleting the selected content.

Here is a detailed explanation of the answer:

1. Extended selection mode:
- Extended selection mode is a feature in many text editors and spreadsheet programs that allows you to select a block of text or data using the arrow keys. This mode is useful when you want to select a rectangular or non-contiguous block of content.

2. Disabling extended selection mode:
- To disable extended selection mode, you can press the Esc key. This will exit the extended selection mode and return the program to the normal selection mode.
- The Esc key is usually located at the top left corner of the keyboard, and it is often labeled "Esc" or "Escape".

3. Other options:
- The other options mentioned in the question (F8, Del, and Enter) are not used to disable extended selection mode.
- Pressing F8 again (option a) will not disable extended selection mode. Instead, it will toggle the mode on or off. If it is currently enabled, pressing F8 again will disable it. If it is currently disabled, pressing F8 will enable it.
- Pressing Del (option b) will delete the selected content, but it will not disable extended selection mode.
- Pressing Enter (option d) is usually used to confirm a selection or execute a command, but it will not disable extended selection mode.

In conclusion, to disable extended selection mode, you should press the Esc key. This will exit the mode and allow you to return to normal selection operations within the program.