The correct answer is a eukaryote. While it would be an impulse to answer prokaryote since the first thing that will come to our minds if we deal with a microorganism that thrives in extreme environments (i.e. hot thermal vents) will be archea, which are prokaryotes. But in this case, it is clearly specified that the single-celled organism has a nucleus which is the characteristic feature of eukaryotes.
Bacteria is incorrect as bacteria are prokaryotes. Plant is incorrect as all plants are multicellular organisms.
Oxygen is needed for cellular respiration to release the energy from food molecules. ... Animals have lungs or gills that allow oxygen to enter the animal's cells while plants have small pores called stoma that allow gases to enter and leave their leaves.
Answer and explanation :
The coral reefs are endangered or exposed by a variety of factors that are hurricane which is a natural phenomenon, diseases and local threats which includes destructive fishing techniques , pollution and global effect of climate change. These pollution is mainly due to sewage and agriculture fishing is destructive because for fishing they usually use cyanide which is very dangerous these are the modern coral reef endangered.
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Prokaryotes are the same as bacteria. So the answer is c., bacteria.