The following question was given in FIITJEE study material:
At first, I thought it should be elimination. But, soon I realized that there is no heating, plus a primary alkyl halide is used. So, "no elimination".
Then, I thought it would proceed through $\ce{S_N2}$, and hence (B) should be correct, as it involves concerted mech.. But, again I was wrong. Why is it so?
The correct answer given is:
(D)