The relatively static lattice in a diamond ensures that the scattering is at a minimum and the thermal conductivity is exceptional good.

The most appropriate phrase to replace the phrase given in bold is “is exceptionally”. It is to be noted that ’good’ is an adjective, so ’exceptional’ should be replaced by ’exceptionally’. ’Exceptional’ is itself an adjective and we all know that an adjective never defines another adjective, but an adverb does. Since option 4 is in the precise grammatical syntax, it becomes the most suitable answer choice.