Around 99% of all species which amounts to over 5 billion species which ever lived on Earth are said to be extinct. The current number of species present on Earth is estimated to be 10-14 million. Hence option A is correct.

The species (including animals, plants, fungi, etc.) which are vulnerable to endangerment in the near future are known as threatened species. The International Union for Conservation of Nature (IUCN) is the authority on threatened species, and it treats threatened species as a group of three categories, depending on the degree relative to which they are threatened. Those categories are Vulnerable species; Endangered species and Critically endangered species. 

The tallest trees in the world are redwoods (Sequoia sempervirens), which tower above the ground in California. These trees can easily reach heights of 300 feet (91 meters).

Caspian tiger has undergone anthropogenic extinction. The anthropocene extinction, is the ongoing extinction event of species during the present holocene epoch, mainly as a result of human activity. It is also called as the holocene extinction or as the Sixth Extinction. Reasons for such extinction include land development, overexploitation, species translocations and introductions, and pollution.
So, the correct answer is 'Caspian tiger'.

Extinction is the complete demise of a species. It takes place when all individuals of a species die out. Extinction has occurred throughout the history of life on Earth. It is the ultimate fate of all species. It has been estimated that 99.9% of all species that have ever lived on Earth are now extinct. The causes of extinction are many and highly variable. They vary from environmental changes brought about by events such as floods, volcanic eruptions, or meteorite impacts, to fluctuations in sea level or climate, to competition between species.

When there are no more individuals of a species (plant or animal) alive anywhere in the world, the species is said to be extinct. Extinction is also known as the process of dying out. The process is a completely natural part of evolution. But sometimes extinctions happen at a higher and a faster rate than usual. Factors such as large body size, small population, and position at a higher trophic level favour extinction. Fear of extinction of species from tropical forests is at the rate of 2-5 species/hr.

The Galapagos islands are known for their large number of endemic species and were studied by Charles Darwin during the second voyage of HMS Beagle. His observations and collections contributed to the inception of Darwin's theory of evolution by means of natural selection. The islands are located in the eastern Pacific Ocean, near the west coast of South America.

An invasive species is a species that is not native to a specific location (an introduced species), and that has a tendency to spread to a degree believed to cause damage to the environment, human economy or human health. The criteria for invasive species has been controversial, as widely divergent perceptions exist among researchers as well as concerns with the subjectivity of the term "invasive". Several alternate usages of the term have been proposed. The term as most often used applies to introduced species (also called "non-indigenous" or "non-native") that adversely affect the habitats and bioregions they invade economically, environmentally, or ecologically. Such invasive species may be either plants or animals and may disrupt by dominating a region, wilderness areas, particular habitats, or wildland–urban interface land from loss of natural controls (such as predators or herbivores).

Species a group of closely related organisms that are very similar to each other and are usually capable of interbreeding and producing fertile offspring. In reproductive isolation refers to the inability of related species to breed. In species, the only same member of an organism can interbreed due to reproductive isolation.

Sickle cell anemia is an inherited blood disease where red blood cells show an abnormal crescent (or sickle) shape. It is an autosomal recessive disease in which one gene of allele codes for a normal protein and the other one codes for defective hemoglobin (sickle-celled).