A proposal is best defined as:

A proposal is best defined as a formal offer to sell. This definition emphasizes the structured and often legally-binding nature of a proposal, especially in the context of business and sales. When businesses create proposals, they typically outline terms, pricing, services, and the specific value they can provide to a potential client or buyer. The formal aspect of a proposal makes it more than just a casual suggestion; it conveys seriousness and intent to engage in a business transaction.

In a business context, proposals are integral in negotiating deals and are crucial when firms seek to secure contracts or clients. While other options suggest collaboration, funding, or a detailed plan, these elements can be components of a proposal rather than its primary definition. Proposals can indeed include suggestions for future collaboration or specific terms and conditions, but the core idea is that they function as formal offers that help facilitate business transactions.