Which of the following is an informal procurement method?

The option indicating "informal, open market" is recognized as an informal procurement method because it involves purchasing goods or services without the structured processes and formalities required in competitive bidding. This method allows for more flexibility and less documentation compared to formal procurement methods.

In informal procurement, purchases can be made quickly and often do not require a requirement for detailed specifications or a tender process, making it suitable for smaller orders or items that are easily obtainable. It operates on more casual terms, relying on vendor relationships and immediate availability rather than a formalized buying process.

On the other hand, group/co-op purchasing is typically a formalized approach where multiple organizations join together to leverage their purchasing power. Formal, competitive bid buying is clearly a structured process where suppliers are invited to submit bids under specified conditions. Centralized purchasing refers to a system where a single department manages all purchasing activities, which also follows a more authoritative and systematic procedure.

Thus, the informal, open market option stands out for its directness and agility in the procurement process, aligning with the characteristics of informal procurement methods.