The primary duty of the teacher is to

Aims of teaching are commonly grouped into narrow, domain-specific objectives (cognitive, physical, moral, social, and so on) and one comprehensive aim — the all-round development of the learner — under which every narrower objective operates as a contributing facet rather than a rival primary aim.

The stem asks for the teacher’s primary duty, that is, the overarching purpose that governs teaching, not one narrow strand of it. Among the given options, "help all round development of the students" names this comprehensive aim directly, since it covers the cognitive, physical, social, emotional, and moral growth of the learner together.

• "raise the intellectual standard of the students" addresses only the cognitive/academic domain of growth.

• "improve the physical standard of the students" addresses only the physical/health domain of growth.

• "imbibe value system in the students" addresses only the moral/ethical domain of growth.

Each of these names one facet that the comprehensive aim already includes, so none can stand alone as the teacher’s primary, overarching duty.

Hence, "help all round development of the students" is the primary duty of the teacher.