In Advaita Vedānta, the true nature of the jīva is identical with Brahman (A).
The perceived difference between jīva and Brahman is only empirical (vyāvahārika) and arises due to limiting adjuncts (upādhi) such as body, mind, and senses—hence (B).
Since the essence of the jīva is Brahman alone (which is the only ontological reality), it is acceptable to say the jīva is ontologically real and one with Brahman when viewed in its true nature—thus (C).
Option (D) is incorrect because Brahman is partless (niravayava); nothing can be a "part" of Brahman.
\[
\boxed{\text{Therefore, (A), (B), and (C) are correct; (D) is not.}}
\]