One advantage to using nuclear power is
a the process of correctly disposing of the waste
b the threat of an accident
c the expense of constructing nuclear reactors
dit produces no air pollution

"NUCLEAR POWER" produces "no air pollution".

Option: D

Explanation:  

As nuclear power does not emit any hazardous gas or smoke thus it produces no air   pollution. But disposing of the waste is still a big issue in the case of nuclear power for which usually underground disposal in appropriate container method is opted but it's too have many disadvantages. Threat of an accident and the expenses of constructing nuclear reactors are really high so other three options are disadvantages of nuclear power.

a:

by the combination of an unlimited number of elements