Which equilibrium is not rapid at room temperature?

In assigning $R-S$ configuration,which among the following groups has the highest priority?

Total number of $\alpha$-hydrogen in the given compound is

The correct energy profile for amine inversion and the hybridization of nitrogen in the transition state is:

Which of the following structures represents the lowest-energy form of $(1S, 2S, 4R)-1,2,4$-trimethylcyclohexane?

In the projection formula $[A]$,two groups ($Br$ and $CH_3$) at the chiral center are interchanged to obtain formula $[B]$. When two groups ($C_2H_5$ and $Cl$) of $[A]$ are interchanged,the projection formula $[C]$ is obtained. Which of the following statements is $NOT$ correct for $[A]$,$[B]$,and $[C]$?