b
Aprotic solvents like \(DMF\) increases the reactivity of nucleophile and favours \(S_{N^{2}}\) reaction. The relative reactivity of alky! halides towards \(S_{N^{2}}\) reactions is as follows

\(C H_{3}-X>\) Primary \(>\) Secondary \(>\) Tertiary

However, if the primary alkyl halide or the nucleophile/base is sterically hindered the nucleophile will have difficulty to getting the back side of the \(\alpha\) - carbon as a result of this, the elimination product will be predominant.

Here, \(C H_{3} C H_{2} B r\) is the least hindered, hence it has the highest relative rate towards \(S_{N^{2}}\) reaction.