Step 1: Understand BoP deficit adjustment.
When a country faces a BoP deficit, the central bank uses its official reserves (foreign currency reserves) to finance the gap.
Step 2: Eliminate other options.
- Portfolio investment: Cross-border purchase of financial assets, not reserve use.
- Net Invisibles: Refers to services, remittances, and income flows.
- Net Factor Income: Wages, interest, and profit from abroad.
Step 3: Apply.
Thus, when RBI (or any central bank) sells foreign currency to bridge deficit, it is called Official Reserve Sale.
Final Answer:
\[
\boxed{\text{Official Reserve Sale}}
\]