An expert means D) The person has a high degree of knowledge about a topic.
The fact that someone published many books or articles does not signify their expertise in the field. They could be knowledgeable, but this is not the same.
Having a college degree does not making someone an expert, it takes 10,000 hours of focus on one subject to become an expert. Although going to college takes more then 10,000 hours, that time is not spent dedicated to learning that specific subject, but rather an array of subjects.
Working in the same job for a long time does not show expertise. People get locked into a job for many reasons. Doing the same thing over and over can help growth, but only to a certain extent.
Therefore, D is the only right answer.
I hope this helps! :)
