Which of the following expenses is deductible for tax purposes?

The choice of unreimbursed employee expenses is correct because it refers to costs incurred by employees in the course of their job that are not reimbursed by their employer and can be deducted from their taxable income. This deduction typically applies to expenses that are ordinary and necessary for the performance of the job. However, it is important to note that changes enacted by the Tax Cuts and Jobs Act of 2017 have suspended the deduction for unreimbursed employee expenses for tax years 2018 through 2025 for most employees.

For the other options, safety shoes purchased by a plumber can be deductible if they meet specific criteria, but eligibility varies depending on the employment structure and company policies. Bottled water purchased by a gig driver for passengers is generally considered a personal expense, which is not deductible. The tax return preparation fee paid by a nonemployed retiree also isn't deductible for the same reason; personal expenses of non-employees are usually not eligible for deductions under most circumstances post-2017 tax reform.

Thus, the context of the taxable employee status underlines why unreimbursed employee expenses is recognized as a potential deduction while considering the limitations set by tax law amendments.