Read the passage given below and answer the following questions:
Nucleophilic substitution reactions are of two types; substitution nucleophilic bimolecular (SN²) and substitution nucleophilic unimolecular (SN¹) depending on molecules taking part in determining the rate of reaction. Reactivity of alkyl halide towards SN¹ and SN² reactions depends on various factors such as steric hindrance, stability of intermediate or transition state and polarity of solvent. SN² reaction mechanism is favoured mostly by primary alkyl halide then secondary and then tertiary. This order is reversed in case of SN¹ reactions.

Tertiary alkyl halides are practically inert to substitution by SN²mechanism because of
(a)  (b)  (c) (d) .