What does 'Red' indicate in the $IUCN$ Red List $(2004)$?

The $IUCN$ Red List $(2004)$ documents the extinction of $784$ species in the last $500 \; \text{years}$, including:

According to $IUCN$,when a taxon is facing an extremely high risk of extinction in the immediate future,it is categorized as:

The impacts of loss of biodiversity may lead to:
$I$. Lowered resistance to environmental perturbation
$II$. Decrease in plant production
$III$. Increased variability in ecosystem processes like water use,pest/disease cycle,and plant productivity
$IV$. Increase in plant production
Choose the correct option.

Write an account on the loss of biodiversity in the world.

The last $20$ years have witnessed the disappearance of $.............$ species.