Which of the following is most reactive towards S_N1 reaction?

C₆H₅C(CH₃)C₆H₅Br

The \(S_N1\) reaction is a two-step process that involves the formation of a carbocation intermediate. The stability of the carbocation intermediate is important for the rate of the reaction. The carbocation intermediate in \(C_6H_5C^+(CH_3)C_6H_5Br\) is stabilized by the electron-donating effect of the two phenyl rings and the methyl group. This makes the carbocation intermediate more stable and the reaction more likely to proceed by the \(S_N1\) mechanism.

The two phenyl rings have a -R effect, which means that they withdraw electrons from the carbocation, making the positive charge less concentrated. The methyl group has a +I effect, which means that it donates electrons to the carbocation, making the positive charge less concentrated. Both of these effects contribute to the stability of the carbocation intermediate.