1. Identify the reaction asked

We need the reagent that converts an alkyl halide, say $R-X$, into an alkyl isocyanide $R-N\equiv C$.

2. Understand ambident nucleophiles

The cyanide group is an ambident nucleophile, meaning it can attack through two atoms:

• through carbon to give nitrile: $R-C\equiv N$
• through nitrogen to give isocyanide: $R-N\equiv C$

3. Effect of the cation in cyanide reagent

• With ionic cyanides like $\mathrm{KCN}$ and $\mathrm{NaCN}$, the cyanide ion is mostly free in solution.
• The carbon end is more nucleophilic, so reaction occurs mainly via carbon attack, giving alkyl cyanide (nitrile):

Similarly for $\mathrm{NaCN}$.

So:

• A: $\mathrm{KCN}$ gives nitrile, not isocyanide.
• C: $\mathrm{NaCN}$ gives nitrile, not isocyanide.

4. Why $\mathrm{AgCN}$ gives isocyanide

$\mathrm{AgCN}$ is largely covalent. In it, the carbon end is more strongly associated with silver, so nucleophilic attack occurs preferentially through nitrogen, producing isocyanide:

Thus alkyl halide is converted into alkyl isocyanide by $\mathrm{AgCN}$.

5. About $\mathrm{NH_4CN}$

$\mathrm{NH_4CN}$ is not the standard reagent used for this conversion in this context; the expected reagent for isocyanide formation is $\mathrm{AgCN}$.

6. Evaluate options

• A: $\mathrm{KCN}$ $\rightarrow$ alkyl cyanide, incorrect
• B: $\mathrm{NH_4CN}$ $\rightarrow$ not the correct reagent here, incorrect
• C: $\mathrm{NaCN}$ $\rightarrow$ alkyl cyanide, incorrect
• D: $\mathrm{AgCN}$ $\rightarrow$ alkyl isocyanide, correct

Final Answer

So the correct option is D.