Within biological communities, some species are important in determining the ability of a large number of other species to persist in the community. Such species are called

Answer: (a) Keystone species

A keystone species has a far larger effect on its community than its numbers suggest (e.g. figs, sea otters, elephants).

The idea

In a stone arch the keystone is the one stone at the top that locks the others; pull it out and the arch falls. A keystone species plays that part in a community: it may be few in number, but many other species depend on it.

Sea otters eat sea urchins → Remove the otters → Urchins multiply and eat the kelp → Kelp forest and its fish disappear

Why the others are wrong

(b) Allopatric species live in separate areas.

(c) Sympatric species share the same area.

(d) A threatened species is at risk of extinction — it says nothing about its role.

Also know

Other examples: fig trees, which fruit all year and feed many animals; elephants, which open up forest and spread seeds. A flagship species is the popular face of conservation (tiger, panda). An indicator species shows the health of a habitat (lichens, frogs).