Which statement is an opinion?

acanthaster planci, also known as the crown-of-thorns starfish, is a carnivorous starfish that preys on coral.
in the reefs off the north coast of jamaica, one can find a species of monatrea annularis, which is flat to capture light efficiently.
stony coral is one example of the hexacoral polyps and has tentacles in groups of six.
people must take action now to protect coral reefs by reducing pollution and emissions of noxious gases.

People must take action now to protect coral reefs by reducing pollution and emissions of noxious gases.

The other options are all facts. They would be found in any encyclopedia or article about stony coral, Monatrea annularis, or Acanthaster planci. The last statement, however, is an opinion. The author uses the word "must" which makes it an opinion statement. If the author stated that reducing pollution and emissions of noxious gases would protect coral reefs, that would be fact. However, the use of the word must creates a demanding tone that is one of opinion.