Social movement organizations often struggle to mobilize supporters from allied movements in their efforts to achieve critical mass. Organizations with hybrid identities—those whose organizational identities span the boundaries of two or more social movements, issues, or identities—are vital to mobilizing these constituencies. Studies of the post-9/11 U.S. antiwar movement show that individuals with past involvement in non-anti-war movements are more likely to join hybrid organizations than are individuals without involvement in non-anti-war movements. In addition, they show that organizations with hybrid identities occupy relatively more central positions in inter-organizational contact networks within the antiwar movement and thus recruit significantly more participants in demonstrations than do non hybrid organizations.

After reading all the lines of the paragraph, it is evident that social movement organisations depend on organisations with hybrid identities because , within them they contain individuals with multiple points of views on different movements and issues. The example gives shows how people who are campaigning for non-anti-war movements are more likely to join hybrid organisations. It also says that organisations with hybrid identities occupy more central positions within organisations that are involved in anti-war movements.

Option A is incorrect. The passage talks more about why hybrid organisations are vital to providing participants in social movements. 

Option C is just an illustration of the idea that the paragraph is trying to make. Hence, it would not be the most appropriate summary of the paragraph.

Option D is incorrect. From the paragraph we can infer that hybrid organisations attract individuals that are deeply involved in non-anti-war movements.

Option B is an appropriate summary of the passage and hence it is the correct answer.