Answer: a. human relations
Step-by-step explanation:
Zappos saying that they hope their company will be the last company the employees work for is them saying that they hope their employees will find working in Zappos constantly alluring such that they would never want to leave (at least until retirement).
It can therefore be inferred from this that Zappos puts in the necessary effort to retain its employees by focusing on human relations in its operations such that employees fill more motivated and fulfilled at Zappos.