When sea otters became extremely rare on the California coast, their usual prey (sea urchins) became overabundant and consumed most of the kelp that was the principal habitat for a diversity of fish species and invertebrates.Which statement best describes the situation?

Keystone species can be defined as those species that are essentially required in a food chain or food web so that the balance in the number of individuals of each of the other species involved in the food chain or food web remains regulated. They help to regulate the balance in the ecosystem.
In the given question, the kelp ( sea-weed ) habitat in California is inhabited by sea-urchins, sea otters, multiple invertebrates and fishes.
Here, the sea-urchins are the prey for the sea otters which are the predators.
The reduction in the population of sea otters prevents their prey from getting hunted by them.
Also, the sea urchins are not preyed upon by other predators.
As a result, the population of sea urchins increases in the kelp habitat, far more than normal.
They tend to occupy the habitat and niche of the invertebrates and fishes.
Hence, they disturb the ecosystem's balance.