With reference to learning unit $12,$ the term barrier system can be defined as:

a$.$

A set of interacting, human, nontechnical and organisational elements that make up the barrier function.

b$.$

A set of interacting, human, technical and organisational elements that make up the barrier function.

c$.$

A set of unrelated, nonhuman, technical and organisational elements that make up the barrier function.

d$.$

A set of unrelated, human, technical and organisational elements that make up the barrier function.