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a) the cristae divides the mitochondria into 2 compartments, the inner matrix and the outer compartment. This concentrates the enzymes of the Krebs cycle in one place (matrix) where they can do their job.
b) the outer compartment affords a place for H+ ions (protons) to accumulate during chemiosmosis, creating a hydrogen imbalance across the cristae membranes. As the protons diffuse back into the matrix, their energy can be used to create lots of ATP.
c) the internal membrane, or cristae is greatly folded. This creates a greater surface area. There are thus more electron acceptors, hydrogen pumps, and ATP synthase enzymes so that electron transport can occur more quickly.
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