Final answer:
The correct word to complete the sentence is 'and,' which is used to join two groups that the promotion policy affects: new and existing employees.
Step-by-step explanation:
The correct word to fill in the blanks in the sentence "The new promotion policy is applied to both new ____________. ____________...existing employees." is "and". Thus, the complete sentence should read: "The new promotion policy is applied to both new and existing employees."
This sentence structure is coordinating two groups that are affected by the policy: new employees and existing employees. The conjunction "and" is used to join these two related groups together, reinforcing that the policy impacts both groups equally.