Which term is used to describe someone who is not able to learn or develop skills at the same rate as most others due to a brain issue?

The correct term to describe someone who is not able to learn or develop skills at the same rate as most others due to a brain issue is "learning disabled." This term encompasses a range of specific learning disorders and does not imply a reduction in overall intelligence, but rather focuses on difficulties in specific areas such as reading, writing, or math that are inconsistent with the individual's overall cognitive abilities.

The other terms provided do not accurately describe the context of learning ability in relation to brain function. "Mildly retarded" is an outdated and offensive term, and the preferred terminology today emphasizes varying levels of intellectual disabilities or developmental delays. "Developmentally delayed" could refer to a broad range of delays in physical, cognitive, or emotional development, but it is not specific to learning issues associated with brain function. "Mentally ill" refers to mental health disorders and does not imply a specific learning disability. Thus, "learning disabled" is the most appropriate term in this context.