Correct option is C
WHO/UNICEF recommend zinc for diarrhea: 10 mg/day for infants <6 months and 20 mg/day for children ≥6 months, for 14 days.
Explanation of options: (a) ✘ 10 mg × 7 days is under-dose and too short duration. (b) ✘ 10 mg × 14 days correct only for <6 months, not for 3-year-old. (c) ✔ Correct; 20 mg × 14 days is the guideline for ≥6 months. (d) ✘ 20 mg twice daily is excessive, not recommended.