It is the first layer of atmosphere .. and the others are major domains

**Explanation:**

The major domains of the Earth include the Troposphere, Biosphere, Lithosphere, and Hydrosphere. The troposphere is the lowest layer of the Earth's atmosphere, extending from the surface up to about 8-15 kilometers above sea level. It is the layer where weather occurs and where most of the Earth's atmospheric mass is located.

**Troposphere**
- The troposphere is characterized by decreasing temperatures with increasing altitude, and it is where weather phenomena such as clouds, rain, and wind occur. It is also where the majority of the Earth's air mass is concentrated.

**Biosphere**
- The biosphere refers to the part of the Earth where life exists. It includes all living organisms, from microscopic bacteria to plants, animals, and humans. The biosphere encompasses various ecosystems, such as forests, grasslands, deserts, and oceans, where different organisms interact with each other and with their environment.

**Lithosphere**
- The lithosphere refers to the solid outer layer of the Earth, which includes the crust and the uppermost part of the mantle. It is composed of various geological materials, such as rocks, minerals, and soil. The lithosphere is divided into tectonic plates that float and move on the semi-fluid asthenosphere beneath them, leading to geological phenomena like earthquakes, volcanoes, and mountain formation.

**Hydrosphere**
- The hydrosphere refers to all the water on Earth, including oceans, seas, lakes, rivers, groundwater, and even water vapor in the atmosphere. It is a critical component of the Earth's system, playing a vital role in regulating climate, supporting various ecosystems, and providing resources for human activities.

**Conclusion**
- Therefore, the correct answer is option 'A' - Troposphere. The troposphere is a major domain of the Earth and plays a crucial role in weather and atmospheric processes. It is not the major domain of the Earth.