I would say that t-butyl benzene would have the highest electron density as the alkyl group attached to it is bulkier than methyl and ethyl thus it will exert a greater +I effect pushing the electrons into the ring making it electron rich. Using same analogy ethyl benzene would have a greater electron density in the ring compared to methyl benzene.

However that is not the case. Something counter intuitive seems to be happening here. The order is methyl benzene >ethyl benzene >t-butyl benzene....

I think this phenomenon can somehow be explained using hyper conjugation, but i don't get it how? Hope it helpful....

Introduction:
In order to determine the electron density at the benzene ring in t-butylbenzene and methylbenzene, we need to analyze the electron-donating and electron-withdrawing effects of the substituents attached to the benzene ring. The electron density at the benzene ring can be affected by the presence of substituents due to their inductive and resonance effects.




Electron Density in t-butylbenzene:
In t-butylbenzene, the tert-butyl group (-C(CH3)3) is attached to the benzene ring. This group has a strong electron-donating effect due to the presence of three methyl groups. The electron-donating effect of the tert-butyl group increases the electron density at the benzene ring.




Electron Density in methylbenzene:
In methylbenzene, a single methyl group (-CH3) is attached to the benzene ring. The methyl group also has an electron-donating effect, but it is weaker compared to the tert-butyl group. Therefore, the electron density at the benzene ring in methylbenzene is lower compared to t-butylbenzene.




Reasoning:
The electron-donating effect of the substituent on the benzene ring is mainly determined by the presence of alkyl groups. Alkyl groups, such as methyl groups, have a positive inductive effect, which leads to the donation of electron density towards the benzene ring. In the case of t-butylbenzene, the tert-butyl group has a stronger electron-donating effect compared to a single methyl group in methylbenzene.




Conclusion:
Based on the electron-donating effects of the substituents, t-butylbenzene has a higher electron density at the benzene ring compared to methylbenzene. The presence of the tert-butyl group in t-butylbenzene, with its stronger electron-donating effect, leads to an increased electron density at the benzene ring.