The correct answer is (C) INB.
A broker's registration number begins with the letters 'INB', and that of a sub-broker begins with 'INS'.
Source: Chapter 4, Section 4.1.2 (Stock Trading)
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The passage explicitly states: "A broker's registration number begins with the letters 'INB' and that of a sub broker with the letters 'INS'." Students often confuse INB (broker) with INS (sub-broker) — remember B for Broker, S for Sub-broker.