Buddhist Schools

Step 1: Understanding the statement.
The statement means that even if a monk attains Arhatship, he cannot reach the supreme category of a Buddha.
Step 2: Sarvāstivādin belief.
The Sarvāstivādins made a clear distinction between the state of an Arhat and that of a Buddha, considering Buddhahood to be superior and not attainable by an Arhat.
Step 3: Analysis of options.
- Sautrāntika: Known for rejecting Abhidharma, not this doctrine.
- Andhaka: Sect with different doctrinal disputes.
- Hinayānī: General term, not specific to this teaching.
- Sarvāstivādī: Correct, they held this doctrine.
Step 4: Conclusion.
Thus, the concept belongs to the Sarvāstivādī school.