What must a partnership provide to each partner annually regarding their share of income?

A partnership must provide each partner with a Schedule K-1 annually to report their share of income, deductions, and credits from the partnership. The K-1 serves as a vital document that outlines each partner's proportionate share of the partnership's income or loss, ensuring that all partners can accurately report this information on their individual tax returns.

Schedule K-1 is particularly important because it helps maintain the transparency of income distribution among partners and ensures compliance with tax laws. Each partner uses the information provided in the K-1 to correctly report their income, which is essential for determining their tax liability.

The other options, while related to partnerships in some manner, do not fulfill the specific requirement of reporting individual income share within a partnership context. A summary of expenses does not provide the necessary details about income distribution, Form W-2 is related to employee wages and does not apply to partners, and a partnership agreement outlines the terms of the partnership rather than the income distribution. Therefore, the requirement for partnerships to provide a Schedule K-1 to their partners aligns directly with IRS reporting standards and the need for accurate individual tax reporting.