The explanation for the answer is as follows:

1. Cepheids & Giant clouds of dust and gas in space: This is incorrect. Cepheids are actually types of stars that pulse in brightness, not giant clouds of dust and gas.

2. Nebulae & Stars which brighten and dim periodically: This is incorrect. Nebulae are indeed the giant clouds of dust and gas in space, symbolizing a sort of nursery for new stars. They are not stars that brighten and dim periodically.

3. Pulsars & Neutron stars that are formed when massive stars run out of fuel and collapse: This statement is correct. Pulsars are rapidly rotating neutron stars, which are the remains of massive stars that have gone supernova.

Hence, only one of the three pairs, namely the Pulsars, is correctly matched within its given description. So, the correct answer is "1 Only one".