According to the International Union for Conservation of Nature and Natural Resources (IUCN) in 2004, how many plant and animal species have been described so far?

Slightly more than 1.5 million

The correct answer is a) Slightly more than 1.5 million.

According to the International Union for Conservation of Nature and Natural Resources (IUCN) in 2004, slightly more than 1.5 million plant and animal species have been described so far. These are species for which there are published records of their discovery and scientific descriptions. This includes species of plants, animals, fungi, and microorganisms that have been identified and named by taxonomists and biologists through rigorous scientific study and documentation.

It's important to note that the number of described species may have increased since 2004 due to ongoing scientific research and discoveries. However, the total number of species on Earth is likely to be much higher since there are many species yet to be discovered and formally described. The actual total number of species on Earth remains uncertain and is a subject of ongoing scientific investigation and estimation.