Answer: d. resources and capabilities are highly mobile across firms.
Step-by-step explanation:
Resource-based theory posits that valuable resources which are difficult to copy are valuable to the success of an organization. Such resources can further develop the capabilities of a firm. In the resource-based model, skilled employees are key to the success of the organization.
A why assumption of this model is that the differences in resources and capabilities bring about competitive advantage.
It should be noted that option D "resources and capabilities are highly mobile across firms" ain't an assumption and is therefore the answer.