What is the major product obtained when $tert-$butyl bromide is heated with silver fluoride?

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$ : Haloalkanes react with $KCN$ to form alkyl cyanides as a main product while with $AgCN$ they form isocyanide as the main product.
Reason $(R)$ : $KCN$ and $AgCN$ both are highly ionic compounds.
In the light of the above statement,choose the most appropriate answer from the options given below:

Which of the following haloalkanes would undergo $S_{N}2$ reaction at the fastest rate?

Increasing order of reactivity of the following compounds for $S_N1$ substitution is:
$(a)$ $(CH_3)_2CH-CH_2Cl$
$(b)$ $CH_3CH_2Cl$
$(c)$ $CH_3O-C_6H_4-CH_2Cl$
$(d)$ $C_6H_5-CH_2Cl$

Which of the following reaction sequences shows an overall inversion of configuration in the product?

Identify the reagent $A$ in the following conversion.
Alkyl halide $\xrightarrow{A}$ Alkyl nitrite