SN1 reactions are nucleophilic substitution reactions that proceed via a two-step mechanism. In the first step, the leaving group leaves the substrate, forming a carbocation intermediate. In the second step, a nucleophile attacks the carbocation to form the final substitution product.

To determine which compound is most reactive towards an SN1 reaction, we need to consider the stability of the carbocation intermediate that would be formed.

Stability of carbocation intermediates can be determined by considering three main factors:

1. Degree of substitution: Carbocations that are more highly substituted (i.e., have more alkyl groups attached) are more stable due to the electron-donating inductive effect of the alkyl groups. This stabilizes the positive charge on the carbocation.

2. Resonance: Carbocations that can undergo resonance stabilization are more stable. Resonance involves the delocalization of the positive charge over multiple atoms, which helps to spread out the charge and stabilize the carbocation.

3. Hyperconjugation: Hyperconjugation refers to the overlap of a filled sigma bond with an empty orbital on a neighboring atom. This electron delocalization can stabilize the positive charge on the carbocation.

Based on these factors, let's analyze the four given compounds:

a) C6H5CH(C6H5)Br: This compound has a secondary carbocation intermediate. It is stabilized by one phenyl group through the inductive effect. However, it does not have resonance or hyperconjugation stabilization.

b) C6H5CH(CH3)Br: This compound also has a secondary carbocation intermediate. It is stabilized by one methyl group through the inductive effect. It does not have resonance or hyperconjugation stabilization.

c) C6H5C(CH3)(C6H5)Br: This compound has a tertiary carbocation intermediate. It is stabilized by two phenyl groups through the inductive effect. It also has resonance stabilization due to the delocalization of the positive charge over the three carbon atoms. Additionally, it can undergo hyperconjugation with the neighboring methyl group. Therefore, this compound is the most reactive towards SN1 reactions.

d) C6H5CH2Br: This compound has a primary carbocation intermediate. It is only stabilized by one phenyl group through the inductive effect. It does not have resonance or hyperconjugation stabilization.

In summary, compound c) C6H5C(CH3)(C6H5)Br is the most reactive towards SN1 reactions due to its tertiary carbocation intermediate, which is stabilized by inductive, resonance, and hyperconjugation effects.