Final answer:
The compound that appears as the most predominant spot on the chromatogram in the given experiment contains the highest number of carbon atoms.
Step-by-step explanation:
In the given experiment, algal cultures were exposed to radiolabeled [14C]O₂ for a brief incubation period and then subjected to 2D-paper chromatography. The compound that appears as the most predominant spot on the chromatogram, as detected by autoradiography, is the one that contains the most carbon atoms.
As the radiolabeled CO₂ is taken up by the algae during photosynthesis, it is transformed into organic compounds. The compound with the highest number of carbon atoms would appear as the most predominant spot on the chromatogram. The exact number of carbons will depend on the specific compounds formed during the fixation of carbon dioxide in the algae.