What type of proposal occurs informally, where a customer asks one vendor to provide a proposal directly?

The correct answer is Informally Solicited Proposal, as this type of proposal arises when a customer engages with a vendor on an informal basis, directly asking them to submit a proposal. The distinction with this type of proposal lies in the informal nature of the request, which usually does not follow the structured procedures that are typical of formal solicitation processes.

In contrast, a Request for Quotation typically involves a formal request for pricing information and can include specific details about quantities and specifications but does not imply an informal interaction like the scenario described. A Request for Proposal is a more formal document that outlines the specific requirements of a project and invites multiple vendors to bid, which is not consistent with the informal nature of the request mentioned. An Invitation for Bids is also a formal process where the proposal is requested based on a predetermined scope and is usually publicized to attract numerous competing offers, further differentiating it from the informal aspect highlighted in the question.

Thus, the informally solicited proposal accurately describes the situation where a direct, informal request is made by the customer to a specific vendor.