Answer: c) backward integration.
Explanation: The scenario best illustrates backward integration which describes a firm purchasing its suppliers in order to make a product itself rather than buying it from another firm. Due to the change in management of the supplier (ThreadTym) which led to increase in prices of the raw materials used by WillowWorks LLC, it would be far more cost efficient to extract the needed raw materials itself than buying at a higher price. So by buying the company ThreadTym, WillowWorks LLC has engaged in backward integration.