Step 1: About Sarvāstivādins.
The Sarvāstivādins were one of the major early Buddhist schools, famous for their Abhidharma tradition.
Step 2: Important text.
Among their works, the Jñāna prasthānam is considered the foundational text of the Sarvāstivādin Abhidharma system.
Step 3: Analysis of options.
- Dharma saṅgrahaṇī: Belongs to other Buddhist scholastic traditions.
- Vinayapitakam: Deals with monastic discipline, not specifically Sarvāstivādin Abhidharma.
- Vigraha vyāvartinī: Philosophical work by Nāgārjuna, not Sarvāstivādin.
- Jñāna prasthānam: Correct, the main text of Sarvāstivādin Abhidharma.
Step 4: Conclusion.
Thus, the important text is Jñāna prasthānam.