Which of the following documents provides full disclosure of all relevant information to investors in a new issue of a limited partnership?

The prospectus is a crucial document in the context of a new issue of a limited partnership, as it serves to inform potential investors about all significant aspects of the investment. This includes detailed information about the nature of the investment, the risks involved, the structure of the partnership, the management team, financial projections, and any pertinent legal considerations.

The prospectus is designed to ensure that investors have a comprehensive understanding of what they are investing in, thereby enabling them to make informed decisions based on full disclosure. This transparency is vital to regulatory compliance, as it protects both the issuer's and the investors' interests by preventing misinformation or lack of information.

In contrast, the other documents listed serve different purposes. A tax opinion typically outlines the tax implications of the investment but does not delve into all relevant aspects. A custody agreement pertains to the safekeeping of assets but is not focused on providing a holistic view of the investment. The partnership agreement lays out the operational framework and responsibilities within the partnership, but it does not serve as a marketing or informational document aimed at potential investors like the prospectus does. Therefore, the prospectus is the document that fulfills the requirement for full disclosure to investors in a limited partnership.