Bryan “not having a backbone” would make it difficult for him to resist peer pressure. Therefore, "subject to a force of influence" would be the best definition.
"Resistant to a force or influence" is incorrect. This is because Bryan’s lack of a backbone implies that he is not resistant to influence.
"To be aware of" is incorrect. This is because having a backbone refers to someone’s ability to stand up to a force, not their awareness of the force.
"Shielded or protected from" is incorrect. This is because lack of backbone would open one up to peer pressure rather than protecting from it.