Swami Vivekananda placed strong emphasis on concentration in education.
Why C fits: Focused attention supports sustained study, reflection, and the ability to grasp an idea deeply.
Why the others differ: Service, professional training, and subject knowledge can all matter in teaching, but concentration is the concept targeted by this question.
Revision point: Concentration is more than memorising information. It helps a learner direct attention, examine an idea carefully, and practise a skill with purpose.