Which value determines if a switch becomes the central point of reference in the spanning tree topology?

Spanning Tree Election Criteria

Spanning Tree builds paths out from a central point along the fastest available links. It selects path according to the following criteria:

1. Lowest root bridge ID (BID)
2. Lowest path cost to the root
3. Lowest sender bridge ID
4. Lowest sender port ID (PID)

Therefore, the answer to your question is Lower Bridge ID