The Red Data Book lists

(a) medicinal plants

(b) threatened/endangered species

(c) common crop varieties

(d) invasive weeds

Correct answer: (b) Threatened / endangered species

The Red Data Book (often called the Red List) is a systematic record of species that are at risk of extinction. It documents plants, animals (and in many lists, fungi) that fall into threat categories such as Critically Endangered (CR), Endangered (EN), and Vulnerable (VU). Each entry typically includes information on the species’ distribution, population trends, major threats, and recommended conservation actions. Countries/regions also maintain their own Red Data Books—for example, India’s Botanical Survey of India (BSI) and Zoological Survey of India (ZSI) publish national assessments.

Why this matters

• Conservation priority: Helps governments and NGOs prioritize habitats and species for protection.
• Policy & law: Informs legal protection, protected-area planning, and impact assessments.
• Awareness & research: Guides funding, scientific studies, and community awareness programs.

Eliminate the other options

• (a) Medicinal plants: Some may be listed if they’re threatened, but the Red Data Book is not a catalogue of medicinal species per se.
• (c) Common crop varieties: Agricultural variety lists are maintained by agricultural boards, not Red Data Books.
• (d) Invasive weeds: These are harmful non-native species; Red Data Books focus on species at risk, not those causing risk.

Final answer: (b) Threatened/endangered species.