Step 1: Recall the main drivers of exchange rate.
Exchange rates are heavily influenced by interest rate differentials, inflation CPI), industrial output, and fiscal/trade policies.
Step 2: Analyze the options.
- Industrial production affects exports/imports, hence impacts currency.
- Interest rates are among the most crucial determinants.
- CPI (inflation) directly affects purchasing power and thus exchange rate.
- GST has little direct impact but still contributes indirectly via trade competitiveness.
- Investor sentiments can influence short-term trends but are not a structural determinant.
Step 3: Conclusion.
The least influential is investor sentiments.
\[
\boxed{B \text{ Investor sentiments}}
\]