Which of the following halides will give $S_N 1$ reaction fastest?

Iodoform on heating with $KOH$ gives

What will be the major product of the following reaction?
$(CH_3)_2CH-CH_2Br \xrightarrow{CH_3O^- / CH_3OH}$

The mechanism of $S_{N}1$ reaction is given as:
$R-X$ $\rightarrow R^{\oplus} X^{\ominus}$ $\rightarrow R^{\oplus} | X^{\ominus}$ $\xrightarrow{Y^{\ominus}} R-Y + X^{\ominus}$
(Ion pair) (Solvent separated ion pair)
$A$ student writes general characteristics based on the given mechanism as:
$(a)$ The reaction is favoured by weak nucleophiles
$(b)$ $R^{\oplus}$ would be easily formed if the substituents are bulky
$(c)$ The reaction is accompanied by racemization
$(d)$ The reaction is favoured by non-polar solvents.
Which observations are correct?

The best method for the preparation of $Me_{3}CCN$ is

Consider the $S_N1$ solvolysis of the following halides in aqueous formic acid:
$(I)$ $CH_3-CH(CH_3)-CH(Br)-CH_3$
$(II)$ $CH_3-C(CH_3)(Br)-CH_3$
$(III)$ $C_6H_5-CH(Br)-C_6H_5$
$(IV)$ $C_6H_5-CH(Br)-CH_3$
Which one of the following is the correct sequence of the halides given above in the decreasing order of their reactivity?