Final answer:
Ribosomes require the greatest speed to form pellets during differential centrifugation because they are the smallest and least dense among the options.
Step-by-step explanation:
In the process of differential centrifugation, different cellular organelles require different centrifugal forces to form pellets due to their varying sizes and densities. Among the options provided, d. ribosomes are the smallest and least dense, thus requiring the greatest speed to sediment and form pellets at the bottom of the tube. This contrasts with larger organelles like nuclei, which pellet at lower speeds due to their larger mass. The order of centrifugation speed needed to pellet these organelles typically follows the tendency for larger and more massive structures to pellet at lower speeds, while smaller and lighter structures like ribosomes require higher speeds to overcome the reduced sedimentation rate due to their diminutive size and mass.