The answer would be:
A: To try to get other people to support a cause or position.
Similar to an advocate which is a person is promoting some sort of cause or position, an advocacy is an activity in which a person or a group of people aim to sway the decisions in many areas such as politics, economics, etc.
If a website is labeled as an "advocacy", then it's purpose is to alter the decisions of the people who click on it to support their cause.
