Answer:
a. To de-emphasize negative news
b. To emphasize an action instead of the recipient of an action
c. To conceal the doer of an action
Explanation:
Passive voice is ideal when you want reduce impact of bad news, to emphasise on an action and not the recipient, and when concealing the doer of an action.
Usually passive voice is used when you want to draw a person's attention from the doer of an action.