The root bridge is the switch with the lowest bridge ID in the spanning-tree instance. The bridge ID comparison starts with priority. If priorities tie, the lower MAC address wins. In many exam exhibits, the correct root is not the switch drawn at the top or center of the topology; it is the switch with the numerically lowest STP identifier. For this exhibit, S2 has the lowest resulting bridge ID, so it becomes the root bridge.
Cisco CCNA 200-301 v1.1 Network Access expects engineers to calculate this cleanly because root selection determines which links forward and which links block. Once S2 is elected, all other switches calculate their lowest-cost path back to S2 and choose root ports accordingly. Design practice is to place the root bridge deliberately, normally near the distribution layer, rather than leaving the outcome to MAC address tie- breakers. In this question, the election result is S2, which maps to answer B.