In the intrinsic (mitochondrial) pathway of apoptosis, cytochrome c released from mitochondria activates caspase-9 by forming a complex with:

• A FADD (Fas-associated death domain protein)
• B XIAP (X-linked inhibitor of apoptosis protein)
• C Apaf-1 (apoptotic protease-activating factor 1) ✓
• D BCL-2

Explanation

In the intrinsic apoptosis pathway, cytochrome c released from mitochondria binds Apaf-1 and dATP, forming the apoptosome — a heptameric wheel-like complex that recruits and activates procaspase-9, which then cleaves and activates executioner caspases-3 and -7. FADD links death receptors to caspase-8 in the extrinsic pathway; BCL-2 is anti-apoptotic and inhibits cytochrome c release; XIAP inhibits already-activated caspases.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

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