3. Seeking outside opinions on group decisions
'Seeking outside opinions on group decisions' would effectively prevent groupthink from occurring.
Groupthink is something that happens when a group of people reach an agreement without critical thinking or analysis of results or alternatives.
Groupthink occurs where due to social pressure, strong hiearchies, compromise seeking, or other reasons. It leads to ideas and decisions, which, although agreed upon in the group, are still not appropriate.
For this reason, seeking outside views on group decisions, by definition, is a good strategy to prevent the groupthink from happening, because the strategy consists in obtaining ideas from outsiders who make decisions.