Final Answer:
Apoptosis is triggered by a caspase cascade, corresponding to option a.
Step-by-step explanation:
Apoptosis, or programmed cell death, is regulated by a series of events involving caspases—proteases that play a central role in the process. Caspases exist as inactive precursors and are activated through proteolytic cleavage. The activation of an initiator caspase, often triggered by various signals, leads to the activation of downstream effector caspases, initiating the cascade. This caspase cascade results in the orderly breakdown of cellular components and the eventual dismantling of the cell. Options b, c, d, and e are not directly associated with the initiation of apoptosis. While calcium (Ca2+) signaling can contribute to apoptosis, the primary trigger involves caspase activation.
Option A is the answer, accurately identifying the caspase cascade as the trigger for apoptosis.