Step 1: Understanding the Concept:
This question is based on Chapter 15 (Purushottama Yoga) of the Bhagavad Gita. Krishna explains the hierarchy of existence: the Perishable (Kshara), the Imperishable (Akshara), and the Supreme (Uttama Purusha). This is the core of Vedic monotheism as presented in the Gita.
Step 2: Detailed Explanation:
Assertion (A) is a direct translation of the second half of Gita 15.17:
"...yo lokatrayaṃ āviśya bibhartyavyaya īśvaraḥ ||"
It describes the Isvara (Lord) as the one who "enters the three worlds" (lokatrayam avishya) and "sustains/supports" (bibharti) them. This emphasizes the immanence of God—He is not just a distant creator but the sustaining energy within the universe.
Reason (R) provides the first half of the same verse and the overarching title of the chapter:
"uttamaḥ puruṣastvanyaḥ paramātmetyudāhṛtaḥ |"
Krishna explains that there is a "Supreme Person" (Uttama Purusha) who is distinct from the mundane matter and the individual soul. This Supreme Person is explicitly "declared/called" (udahritah) in the scriptures as the Paramatman.
The Reason (R) explains the Assertion (A) because the identity of the Lord as the Paramatman (Reason) is precisely what gives Him the capacity to be the eternal sustainer of the three worlds (Assertion). He sustains because He is the Supreme Being who transcends both the changing and the unchanging. These two statements are the definition and the function of the Purushottama respectively.
Step 3: Final Answer:
Both statements are from BG 15.17. The identity of the Supreme Purusha (R) is the ontological foundation for His sustaining action (A).