Which among the following is the most liberal Open Education Resource license?

CC-BY is the most liberal Creative Commons licence for open educational resources, allowing any reuse as long as the creator is credited.

Under CC-BY, users may copy, adapt and redistribute the work, even commercially, with only attribution required.

CC-BY-SA adds a share-alike condition, forcing derivatives to carry the same licence, making it more restrictive.

CC-BY-ND forbids derivative works, so the material cannot be modified, which is more restrictive.

CC-BY-NC bars commercial use, limiting how the resource may be reused, which is more restrictive.

The fewer conditions a licence imposes, the more open and liberal it is.