Which of the following forms is not required to be sent with California income tax returns?

The correct choice indicates that Form 1099-NEC - Nonemployee Compensation is not required to be mailed with California income tax returns. This form is used to report nonemployee compensation, which is typically income earned by independent contractors or freelancers. While taxpayers must report the income shown on this form when filing their tax returns, there is no requirement to submit the actual form with the return unless specifically requested by the tax authorities.

In contrast, Form 540 is the primary income tax return for California residents and is required. Similarly, Form W-2 reports wages and taxes withheld from an employee's pay, and it must be included with the return to substantiate income for wage earners. Form 1098, which provides information on mortgage interest paid, can also be significant for taxpayers who itemize deductions, and it is usual practice to include this document when relevant.

Therefore, while all forms play a role in reporting income and tax information, the 1099-NEC does not need to be submitted with the tax return, making it the correct answer in this scenario.