Upanishads are a collection of ancient texts that form the basis of Hindu philosophy. They are considered to be the culmination of the Vedas and are highly revered in Hinduism. The correct answer to the question is option 'C', which states that Upanishads were not part of the early Vedic texts. Let's discuss this in detail.

Meaning and Nature of Upanishads:
- Upanishad literally means 'approaching and sitting near'. This term reflects the nature of these texts, which are essentially records of spiritual conversations between teachers (gurus) and students (shishyas).
- The texts contain profound philosophical and metaphysical teachings, often presented in the form of dialogues or discussions.
- Upanishads explore various aspects of human existence, the nature of reality, the self, and the ultimate truth (Brahman).

Relationship with Vedic Texts:
- The Upanishads are not part of the early Vedic texts or the Samhitas, which are the oldest layer of the Vedas.
- The Vedas are a collection of hymns, rituals, and incantations that were primarily concerned with religious and ceremonial practices.
- The Upanishads emerged later as a response to the philosophical and spiritual queries that arose during the Vedic period.
- While the Vedas focus on rituals and external religious practices, the Upanishads delve deeper into the nature of the self, the universe, and the ultimate reality.

Key Features of Upanishads:
- The Upanishads contain profound insights into various philosophical concepts such as karma, dharma, reincarnation, and moksha (liberation).
- They emphasize the importance of self-realization and the realization of the ultimate truth, which is described as Brahman.
- The Upanishads also discuss different paths to spiritual enlightenment, including knowledge (jnana), devotion (bhakti), and meditation (dhyana).
- These texts have greatly influenced not only Hindu philosophy but also other philosophical and spiritual traditions worldwide.

Conclusion:
In conclusion, Upanishads are a collection of ancient texts that contain profound spiritual and philosophical teachings. They are not part of the early Vedic texts but emerged later as a response to the deeper existential queries. Upanishads are highly regarded in Hinduism and have had a significant influence on various philosophical and spiritual traditions.

These were part of the later Vedic texts.