Which of the following is the least stable conformer of cyclohexane?

$A$ compound was synthesized by a student,but its structure was not identified. However,his instructor told him that it was a meso compound with $5$ carbons and $2$ stereogenic centers. Which of the following structures should the student consider as possibilities for his compound?

Which carbocation is the most stabilized?

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

According to $Cahn-Ingold-Prelog$ sequence rules,the correct order of priority for the given groups is

How many stereoisomers are possible for the following molecule?