Formula Recall: NPV has one standard formula worth memorizing directly: PV of Inflows − PV of Outflows.
This matches option 3 exactly — it accounts for the time value of money by discounting future cash flows back to today before subtracting.
Option 1 ignores discounting (uses raw totals), option 2 describes a ratio (more like a Benefit-Cost Ratio), and option 4 skips discounting entirely — none of these is the actual NPV formula.