In a supply chain network, the levels can be categorized into various stages, from raw material suppliers to end customers. The "most upstream external activity" refers to the earliest stage in the supply chain that is external to the organization. Here’s a breakdown of the options:

Supplier to contractor: This represents the activity between the supplier (who provides raw materials) and a contractor (who might process these materials). This is the most upstream activity as it deals with the initial stages of acquiring raw materials.

Manufacturing to supplier: This would imply the flow from manufacturing (internal) back to the supplier, which doesn't fit the context of upstream activity.

Customer to distribution: This is a downstream activity, focusing on moving products closer to the end customer.

Customer to contractor: This is also downstream and focuses on the interaction after the product is finished. Thus, "Supplier to contractor" is the most upstream external activity, dealing with raw material acquisition and initial processing stages.

References

Chopra, S., & Meindl, P. (2016). Supply Chain Management: Strategy, Planning, and Operation.

Mentzer, J. T. (2001). Supply Chain Management.