What is essential regarding the notes to financial statements for a single-employer plan?

The correct answer highlights the importance of reporting the net cash flow annually for a single-employer plan in the notes to the financial statements. This is crucial because it provides stakeholders, such as investors, regulators, and plan participants, with a clear understanding of the financial health of the pension plan. The annual reporting of net cash flow includes all incoming contributions and outgoing benefit payments, which helps assess the sustainability and liquidity of the plan.

Transparent reporting of net cash flow can indicate whether the plan can meet its current and future obligations, allowing users of the financial statements to make informed decisions about the viability of the plan. This data is integral for assessing the plan's performance and making necessary adjustments to ensure financial stability.

In contrast, focusing only on projected liabilities, costs and revenues, or employee contributions would provide a limited view of the plan's financial position and could lead to misunderstandings about its overall health. These other aspects are also important but do not capture the operational cash movements that are critical for assessing liquidity and funding status on an ongoing basis.