The hyperconjugative stabilities of tert-butyl cation and $2$-butene, respectively, are due to

• A

  $\sigma \rightarrow p$ (empty) and $\sigma \rightarrow \pi^{\star}$ electron delocalisations.

• B

  $\sigma \rightarrow \sigma^*$ and $\sigma \rightarrow \pi$ electron delocalisations.

• C

  $\sigma \rightarrow p$ (filled) and $\sigma \rightarrow \pi$ electron delocalisations.

• D

  $p$ (filled) $\rightarrow \sigma^{\star}$ and $\sigma \rightarrow \pi^*$ electron delocalisations.