What is meant by entanglement sudden death?

The correct answer is the abrupt vanishing of entanglement at a finite time, before decay is complete. The release explains that each of two entangled particles tends to slip from the excited state to the ground state the way an atom loses energy to its surroundings, and that the entanglement weakens as this happens. What surprises physicists is that the entanglement can disappear altogether at a definite moment, well before the decay itself has finished, and that abrupt disappearance is what carries the name. Option A describes the decay process itself, which is a different thing and takes longer. Option C describes a readout failure in a quantum computer, a practical engineering problem rather than this phenomenon. Option D describes wave function collapse on measurement, a separate idea of quantum mechanics. The team showed that a single flip applied at the right moment can delay this death or prevent it entirely, which they call avoidance.