Correct Option (a):
Pair 3 is correctly matched.
- Pulsars are highly magnetized, rapidly rotating neutron stars.
- They are formed from the gravitational collapse of the core of a massive star after a supernova explosion, once the star has exhausted its nuclear fuel.
- These objects emit beams of electromagnetic radiation from their magnetic poles, which are observed as regular pulses when the beam sweeps past an observer.
Incorrect Options:
Pair 1 is incorrectly matched.
- Cepheids are a type of variable star that exhibit periodic changes in luminosity due to pulsations in their outer layers.
- They are significant as 'standard candles' for measuring cosmic distances.
- The description "Giant clouds of dust and gas in space" accurately describes nebulae, not Cepheids.
Pair 2 is incorrectly matched.
- Nebulae are vast interstellar clouds composed of dust, hydrogen, helium, and other ionized gases.
- They serve as stellar nurseries where new stars are born, or are remnants of dead stars (e.g., supernova remnants, planetary nebulae).
- The description "Stars which brighten and dim periodically" refers to variable stars such as Cepheids, not nebulae.
Since only one pair (Pair 3) is correctly matched, options (b), (c), and (d) are incorrect.