Final answer:
The correct completion for the sentence is 'Manager/Designated Individual', and it is unethical not to report such significant changes in the workplace.
Step-by-step explanation:
The phrase that completes the sentence "Failure to notify the board of a change of _____ is unethical conduct." is (c) Manager/Designated Individual. This is considered unethical because as an employee, you are expected to communicate significant changes that can affect the management or governance of the business you work for. Not communicating such changes goes against the basic principles of professional conduct in the workplace, which includes respect for your colleagues and the broader integrity of the business operations. In general, employees should always aim to keep relevant parties informed and uphold the values and guiding decisions that underpin the organization's operations.