The question statement refers to a reaction and asks for the major product under an $S_N2$ mechanism, but the actual reaction scheme and option structures are missing in the provided prompt. All four options $A,B,C,D$ are blank, so there is no chemical information available to analyze.

1. What is required to solve this question?

To determine the major product in an $S_N2$ reaction, we must know:

1. the substrate structure,
2. the nucleophile/reagent,
3. the leaving group,
4. the stereochemistry if relevant,
5. the actual product structures/options.

Without these, the product cannot be derived.

2. Why the answer cannot be verified

An $S_N2$ reaction depends strongly on substrate and geometry:

• primary halides usually undergo $S_N2$ readily,
• tertiary centers do not,
• there is backside attack,
• Walden inversion occurs at the reacting stereocenter.

But since the reaction image/options are absent, none of these can be applied to select among $A,B,C,D$.

3. Comparison with stored answer

The stored correct answer is given as $D$, but because the actual reaction and options are missing, I cannot independently verify whether $D$ is correct.

So I must disagree with certainty of validation, not because $D$ is definitely wrong, but because the information is insufficient to solve the problem properly.