Introduction:
The question asks about the number of plant and animal species that have been discovered and described so far. The correct answer is option 'A', which states that 1.5 million species have been discovered and described.

Explanation:
Here is a detailed explanation of why option 'A' is the correct answer:

1. The Known Species:
- Scientists estimate that there are approximately 8.7 million species of plants and animals on Earth.
- However, the exact number is difficult to determine as new species are constantly being discovered, and some may become extinct before they are even identified.

2. Discovery and Description Process:
- Discovering and describing a new species is a complex and time-consuming process.
- Scientists rely on various methods, including fieldwork, DNA analysis, and museum collections, to identify and describe species.
- This involves studying the physical characteristics, behavior, habitat, and genetic makeup of the organisms.

3. Cataloging and Classification:
- Once a new species is discovered and described, it is cataloged and classified into the existing taxonomic system.
- The taxonomic system organizes species into groups based on their evolutionary relationships and shared characteristics.
- This helps scientists understand the diversity of life and how different species are related to each other.

4. Number of Described Species:
- As of now, scientists have described and named approximately 1.5 million species of plants and animals.
- This includes both terrestrial and marine species from various regions around the world.
- The number of described species is constantly increasing as new discoveries are made and existing species are better understood.

5. Unexplored Biodiversity:
- Despite the progress made in discovering and describing species, there is still a vast amount of biodiversity that remains unexplored.
- Many regions, especially in tropical rainforests and deep oceans, are still relatively unexplored, and countless new species are waiting to be discovered.

Conclusion:
In conclusion, the number of plant and animal species that have been discovered and described so far is approximately 1.5 million. However, this represents only a fraction of the estimated 8.7 million species on Earth. The process of discovering and describing species is ongoing, and scientists continue to explore and catalog the Earth's biodiversity.