Which of the following is NOT a factor of differing tax variations?

The correct answer highlights that corporate structure does not directly influence variations in tax rates or policies as significantly as the other factors listed. Tax variations are often influenced by broader contexts that affect entire populations or regions rather than the internal structure of companies.

Geography plays a vital role in tax variations because different locations may have distinct policies based on local government needs, economic conditions, and populations. Geology could indirectly influence tax variations, especially if resource extraction is involved, which contributes to different tax incentives or rates based on resource availability. Demographics also significantly shape tax policy, as different population groups may have varying needs for public services, which in turn influences how local or state taxes are structured.

In contrast, corporate structure primarily pertains to the organization and management of individual businesses rather than to overarching taxation policies that vary by geographic or demographic considerations. Therefore, while corporate structure can impact a corporation's tax obligations, it is not a fundamental factor in the wider context of tax variation across different regions or populations.