C(triangle shape-lol)Major7,
is the correct answer.
Its funny,
i only found this forum
because I typed the following into google;

If Tool's JAMBI focus' mostly on the DMajorFlat7 key,
then what is the I Chord for JAMBI?

Hint: It's been mentioned in this thread recently.

Um,
And if anyone disagrees that jambi is written using DMajorFlat7,
let me know! I havent really been learning it,
just jammed along after doing some basic modal/scale/chord progression study.
Thanks.

So, Again,
Supposing Jambi's main riff is likely performed in the key of DmajorFlat7,
What is the TONIC, or I, or Root, or 1st or Ionion Key of JAMBI: Meaning: What pure Major Scale may you use to solo atop it, or under it, or with it?
Hint #2: its not Dmajor! :)

:)

Supposin'.