What is Kessler Syndrome?

The correct option is D

Proliferation of space debris in low earth orbit.

The “Kessler Syndrome” is an orbital debris term that has become popular outside the professional orbital debris community without ever having a strict definition. The Kessler syndrome (also called the Kessler effect, proposed by the NASA scientist Donald J. Kessler in 1978, is a scenario in which the density of objects in low Earth orbit (LEO) is high enough that collisions between objects could cause a cascade—each collision generating space debris which increases the likelihood of further collisions. One implication is that the distribution of debris in orbit could render space exploration, and even the use of satellites, unfeasible for many generations.