Subjects are generally limited to:

Curriculum theory draws a specific distinction between a "subject" and a "discipline". A subject is a fixed, simplified unit of knowledge (such as Mathematics, Science, or Language) organised into a timetable for teaching and learning at the school stage. A discipline, by contrast, is a specialised, research-oriented body of knowledge pursued by a community of scholars, and it is this term that describes organised study at the college/university level.

Applying this to the question: because "subjects" are the school-level curriculum unit, the sentence "Subjects are generally limited to" is completed by Schools -- schools are precisely the institutions that divide the school day into a fixed set of subjects (e.g., Hindi, English, Mathematics, Science, Social Science).

Each of the other options describes a different way of organising learning, not a subject-wise curriculum:

• Universities: organise learning into disciplines and degree programmes (e.g., B.A., B.Sc., M.A.) that combine several subjects or electives into one specialised, research-oriented field of study.

• Professional courses: are structured around earning a specific professional qualification or license (e.g., engineering, medicine, law, chartered accountancy) through a defined sequence of papers/courses tied to that qualification.

• Vocational courses: are organised around skill modules and trade competencies (e.g., ITI trades) aimed at direct employability in a particular occupation.

Note: in everyday speech, "subject" is sometimes used loosely for a university course too, but the standard curriculum-theory convention -- subject (school) versus discipline (university) -- is what this distinction tests.