Exobiology (also called astrobiology) is the branch of biology in which scientists study which of the following?

Introduction:
As our understanding of the universe grows, scientists are increasingly interested in whether life exists beyond our own planet. This curiosity has led to the development of a specialised branch of biology called exobiology or astrobiology. The question checks whether you can correctly identify the focus of exobiology and distinguish it from other areas of biological study that deal only with life on Earth.

Given Data / Assumptions:
- The key term is exobiology, also commonly known as astrobiology. - Options mention external structure of living beings, life on Earth, life in upper atmosphere and life in outer space. - We assume awareness of space exploration and the search for extraterrestrial life. - The question asks about the main subject matter studied in this branch.

Concept / Approach:
Exobiology or astrobiology is defined as the study of the origin, evolution, distribution and future of life in the universe, with a special focus on life beyond Earth. It includes the search for microbial life on Mars, potential life in the oceans of icy moons like Europa or Enceladus, and theoretical models of life on exoplanets. While it uses many principles of terrestrial biology, its defining feature is the investigation of life in outer space and on other celestial bodies. Therefore, the correct option is the one that explicitly mentions life in outer space, on planets and satellites, rather than just life on Earth or in its atmosphere.

Step-by-Step Solution:
Step 1: Break down the word exobiology: exo means outside and biology means study of life. Step 2: Interpret this as the study of life outside the Earth, that is, in outer space. Step 3: Compare this interpretation with the answer choices and look for the option that talks about life in space, on planets or satellites. Step 4: Recognise that options A and B describe life on Earth only, and option C is limited to upper atmosphere, which is still part of Earth. Step 5: Select the option that states life existing in outer space, including on other planets and satellites, as this matches the standard definition of exobiology.

Verification / Alternative check:
Scientific organisations such as space agencies describe astrobiology as the study of life in the universe, including life on Earth and the possibility of extraterrestrial life. Research programs focus on Mars missions, the study of extremophiles on Earth as models for alien life, and detection of biosignatures in exoplanet atmospheres. These descriptions consistently link exobiology with extraterrestrial life and not merely with external morphology or the Earth's atmosphere, confirming that life in outer space is the central theme.

Why Other Options Are Wrong:
The external structure and anatomy of living beings on Earth: This is covered by anatomy and morphology, not by exobiology. Life present only on the Earth's surface and oceans: This is general biology or ecology that deals with terrestrial and marine life, not specifically exobiology. Life present in the upper layers of the Earth's atmosphere: This could involve aerobiology or microbiology of air, but it still concerns Earth, not life beyond it.

Common Pitfalls:
The word exo may confuse some learners into thinking about external structures rather than external to Earth. Another common mistake is to think that anything unusual such as life in the upper atmosphere must be exobiology, even though it is still within the Earth system. A helpful memory aid is to connect exobiology or astrobiology with space missions, Mars rovers and the search for aliens, which clearly points to life in outer space.

Final Answer:
Exobiology is the study of Life existing in outer space, including on other planets and satellites.