Which of the following molecule has highest electron density in the phenyl ring?

In (a), maximum hyperconjugation takes place increasing the electron density in the phenyl ring to the most. In (c), the two methyl groups bring negative charge on the same carbon by hyperconjugation, hence opposes one another and decreases the extent of hyperconjugation. In (b and d), there is either no or very less electron donation to phenyl ring by hyperconjugation effect.