The food that my grandmother MAKES is incredible.
I agree with this
'that' can be omitted in English
True, that is common practice but grammatically the "that" introduces a phrase within the sentence. Remembering that it should be there helps to remember to put the "que" in the Portuguese sentence.
The food my grandmother makes is incredible.
agree too - in english you make food, not "do" it
Depends on how close you are to your cooking.
As an English speaker, 'the food which my grandmother makes is incredible', is a more traditional (old fashioned) translation. It should be acceptable.
Given we're leaning Portuguese and not English, "which" must also be accepted. May be as a typo if it's so bad