The question asks which compound undergoes the fastest $S_N2$ reaction, but the actual structures/options for A, B, C, and D are missing in the prompt.

Step 1: What determines the rate of an $S_N2$ reaction?

For an $S_N2$ reaction, the rate depends mainly on:

1. Steric hindrance around the carbon bearing the leaving group
   • Methyl $>$ primary $>$ secondary $\gg$ tertiary
2. Leaving group ability
   • Better leaving group gives faster reaction
   • Typically: $I^- > Br^- > Cl^- \gg F^-$
3. Resonance/allylic/benzylic activation
   • Benzylic and allylic halides can react very fast by $S_N2$
4. Substrate structure
   • Vinyl and aryl halides do not undergo normal $S_N2$ easily

So, in general, the fastest $S_N2$ substrate is usually:

• methyl halide, or
• primary benzylic/allylic halide with a good leaving group, depending on the given options.

Step 2: Limitation here

Since the options are blank:

• A:
• B:
• C:
• D:

it is not possible to derive independently which specific compound is fastest.

Step 3: Comparison with stored answer

The stored correct answer is C, but because the structures are missing, I cannot verify whether option C is actually the fastest $S_N2$ substrate.

Therefore, I cannot confidently agree with the stored answer without the compounds being shown.

Conclusion

Please provide the structures/formulas of options A, B, C, and D so I can solve it properly and verify the correct answer.