Final answer:
The substrate affects SN2 reactions. Tertiary substrates do not react by SN2 due to steric hindrance.
Step-by-step explanation:
The structure of the substrate affects SN2 reactions. The order of reactivity is: methyl > primary > secondary > tertiary. Tertiary substrates do not react by SN2 because the nucleophile is prevented from approaching the carbon due to steric hindrance. So, the correct answer is option d) Potassium tert-butoxide, which is a tertiary substrate and does not undergo SN2 reactions with any base.