If you set [C.Fn II -6: Highlight tone priority] to [1:Enable] on your camera, the settable ISO range will be limited to ISO 200-6400, so ISO 100 cannot be selected.
The [D+] icon will be displayed on the LCD panel and in the viewfinder when highlight tone priority is enabled.
If you wish to make the ISO 100 setting, change the setting as follows.
Select [Disable] to change the [C.Fn II -5: Highlight tone priority] setting
1. Set the power switch to ⋖ON⋗.
2. Press the ⋖MENU⋗ button to display the menu.
3. Press the ⋖Cross Keys⋗ to select [Custom Functions(C.Fn)] under the [ ] tab, then press the ⋖SET⋗ button.
4. Press the ⋖Cross Keys⋗ to select Custom Function No.5 [Highlight tone priority], then press the ⋖SET⋗ button.
5. Select [0: Disable], and then press the ⋖SET⋗ button.
6. Press the shutter button halfway, and the camera will be ready for shooting. Confirm that [D+], which indicates Highlight tone priority, is not displayed on the LCD monitor and in the viewfinder.
Caution
About Highlight tone priority