Which of the following statements is correct?

(a) Since 3° carbocation is most stable, the reaction proceeds via the formation of 3° carbocation, i.e. ${\mathrm{S}}_{\mathrm{N}}1$. 

(b) Since 1 ° carbocation is least stable, the reaction does not proceed via the formation of carbocation, i.e. ${\mathrm{SN}}_2$. 

( c) Since there is inversion in optical rotation, the reaction must have proceeded via SN2 mechanism.

 ( d) Since there is no inversion in optical rotation, the reaction must have proceeded by SN 1 mechanism as in ${\mathrm{SN}}_2$ there occurs inversion of configuration of carbon atom.