Which text contains the verses on 'the nature of the self' and 'the true sacrifice' and was composed in 6th-century BCE?

Chhandogya Upanishad

The correct answer is Option (1) → Chhandogya Upanishad

Chhandogya Upanishad contains the verses on 'the nature of the self' and 'the true sacrifice' and was composed in 6th-century BCE.

"The nature of the self:  This self of mine within the heart, is smaller than paddy or barley or mustard or millet or the kernel of a seed of millet. This self of mine within the heart is greater than the earth, greater than the intermediate space, greater than heaven, greater than these worlds."