Explanation:

Cepheids: These are indeed stars that brighten and dim periodically. They have a well-defined relationship between their luminosity and pulsation period, making them very important in determining distances in the universe. So, the description in the pair is incorrect for Cepheids.

Nebulae: Nebulae are giant clouds of dust and gas in space. They can be the birthplaces of stars, or the remnants of a star's explosion. So, the description in the pair is incorrect for Nebulae.

Pulsars: Pulsars are highly magnetized, rotating neutron stars that emit beams of electromagnetic radiation. Neutron stars are formed when massive stars run out of fuel and undergo a supernova explosion. The description in the pair is correct for Pulsars.

Based on the analysis, only the third pair (Pulsars) is correctly matched.