Upanishad are books on A. Religion B. Law C. Yoga D. Philosophy

A) these a important vedic texts

philosophy

The Upanishads, written in India probably between 800 BC and 500 BC, are a collection of texts of religious and philosophical nature. They are a part of the Vedas, ancient Sanskrit texts that contain some of the central philosophical concepts and ideas of Hinduism, some of which are shared with religious traditions like Buddhism and Jainism. Around 200 Upanishads are known, of which the top 10 or so are the oldest and most important and are referred to as the principal or main (mukhya) Upanishads. Mukhya Upanishads can be associated with one of the schools of the four Vedas(i.e. Rig Veda, Yajur Veda, Sama Veda & Atharva Veda)