ediff-diff-options is a variable defined in `ediff-diff.el'.
Its value is

Options to pass to `ediff-diff-program'.
If Unix diff is used as `ediff-diff-program',
then a useful option is `-w', to ignore space.
Options `-c', `-u', and `-i' are not allowed. Case sensitivity can be
toggled interactively using M-x ediff-toggle-ignore-case.

Do not remove the default options. If you need to change this variable, add new
options after the default ones.

This variable is not for customizing the look of the differences produced by
the command M-x ediff-show-diff-output. Use the variable
`ediff-custom-diff-options' for that.

You can customize this variable.