Which of the following compounds is the most stable?

The total number of stereoisomers formed by the given compound is:

$R$ and $S$ configuration of compound $(A)$ & $(B)$ will be

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 $[C]$?

Arrange the following compounds in decreasing order of their acidic strength:

Which of the following options is the correct order of relative stabilities of cations $I$,$II$,and $III$ as shown below (most stable first)?
$(I) \, C_6H_5CH_2^+$
$(II) \, CH_2=CH-CH_2-CH^+-CH_3$
$(III) \, (CH_3)_3C^+$