In the intrinsic pathway of apoptosis, a protein called is released from the mitochondria into the cytosol and binds to the adaptor protein Apaf1, causing it to oligomerize into a wheel-like assembly called an apoptosome, which then recruits initiator caspase-9 proteins.

Cytochrome-c

Explanation:

Apoptosis refers to the genetically controlled pathway which leads to the death of the cell.

The apoptosis is an important pathway as it maintains the shape and size of the organism as well as in the formation of organisms. The pathway involves many proteins like caspase-9, Apaf1 and many others.

The mitochondria organelle is also involved in the pathway as the cell gets damaged, the Bcl-2 family is activated which increases permeability of the mitochondria.

The cytochrome-c of the mitochondria is then released from mitochondria and binds to the Apaf1 domain. This leads to conformation change in the Apaf1 and takes shape of wheel like structure.

Thus, cytochrome-c is correct.