The most effective way for organized groups to shape public opinion is by

The use of the mass media

Explanation:

The term mass media refers to a mean that allows reaching a large amount of people, for example, the newspapers, the radio, the television, the internet, etc.  

The Public opinion consists of the thinking of the majority of the people, concerning government and politics. A big influence in the formation of public opinion can be family and education, however, mass media is really effective in shaping the public opinion because it reaches large audiences.

(using the mass media)  i hope it helps :)

it's correctly