Choice A is the correct answer. Find when charge drops below 80%.

1. Retention: Loses 5%, keeps 95% = 0.95 per day.
2. Formula: $C(t) = 100(0.95)^t$.
3. Test:
   • Day 4: $100(0.95)^4 \approx 81.45\%$ (above 80%)
   • Day 5: $100(0.95)^5 \approx 77.38\%$ (below 80%) ✓
4. Answer: 5 days.

💡 Strategic Tip: Test successive days to find when it crosses the threshold.

Choice B is incorrect because charge is still above 80% at day 4. Choice C is incorrect because charge is well above 80% at day 3. Choice D is incorrect because it drops below 80% before day 6.