1. Concept: ambident nucleophile

An ambident nucleophile is a nucleophile that can attack through two different atoms. Examples:

• $CN^-$ : can attack through C or N
• $NO_2^-$ : can attack through N or O

In such cases, the product may differ depending on the reagent used, especially with silver salts vs potassium salts.

2. Check each pair

(A) $AgCN/KCN$

The nucleophile involved is $CN^-$, which is ambident.

• With $KCN$, ionic character is higher, so attack occurs mainly through carbon, giving nitriles: $R-C\equiv N$.
• With $AgCN$, bonding is more covalent/polymeric, and attack occurs through nitrogen, giving isocyanides: $R-N\equiv C$.

So, (A) is correct.

(B) $RCOOAg/RCOOK$

The nucleophile is carboxylate ion $RCOO^-$. Although it has two oxygen atoms, they are equivalent by resonance and it does not behave as the standard ambident nucleophile giving two distinct constitutional products via different donor atoms in this context.

So, (B) is not correct.

(C) $AgNO_2/KNO_2$

The nucleophile is $NO_2^-$, which is ambident.

• With $KNO_2$, attack occurs through O, giving alkyl nitrites: $R-ONO$.
• With $AgNO_2$, attack occurs through N, giving nitroalkanes: $R-NO_2$.

So, (C) is correct.

(D) $AgI/KI$

The nucleophile is $I^-$, which can attack only through iodine. It is not ambident.

So, (D) is not correct.

3. Correct pair(s)

The correct pairs are:

Hence, the correct option is: