Which syllable in carbonaceous is most heavily stressed?
The most heavily stressed syllable would be "-nace-."
According to Merriam-Webster's website
(Would that be a Merriam Webstersite? Maybe just Webstite? I digress.)
According to their website, the phonetic pronunciation looks like this:
Basically, "nā" gets the stress, and "kär" gets the secondary stress.
This is shown by the apostrophe and the comma coming before these syllables respectively.