The correct answer is option 3, "Endangered plants and animals." The `Red Data Book` is a compilation of information about endangered and threatened species. It provides a comprehensive account of the species that are at risk of extinction or have a high risk of becoming extinct in the near future.

Option 1, "Endangered plants only," is incorrect because the Red Data Book includes not only endangered plants but also endangered animals.

Option 2, "Fossil plants," is incorrect because the Red Data Book focuses on living species and does not include information about extinct plants and animals.

Option 4, "Extinct animals only," is incorrect because the Red Data Book includes information about both endangered and extinct species, not just extinct animals.