Final answer:
The marketing manager has developed and plans to test a research hypothesis.
Step-by-step explanation:
The marketing manager of the bakery has developed and plans to test a B) A research hypothesis. The hypothesis being tested is whether customers will prefer the new whole wheat bread recipe compared to the current recipe. The manager wants to make a batch of the new bread and test it with customers. If more than half of the customers prefer the new recipe, the manager plans to change to the new recipe.