To use special chemicals, smoke or gas to destroy the harmful insects or bacteria in a place

Option A, 'spray' is very general and can be applied to scenarios other than what is mentioned in the sentence 'to use special chemicals, smoke or gas to destroy the harmful insects or bacteria in a place.' Similarly, decontamination can involve much more than using special chemicals, especially since 'substances' identified in option C are different from insects. Both option A and option C are too broad relative to the given context. 

'Fumigate' means to disinfect or purify an area using fumes of certain chemicals, and is the best possible choice among the given options. Option B is the correct answer.