Step 1: Classify severity first. In severe dehydration the standard WHO Plan C protocol is applied with intravenous Ringer's lactate, as oral therapy is inadequate for an emergency.
Step 2: Use the age-specific schedule for a child below one year: give $30\ mL/kg$ rapidly in the first hour, followed by $70\ mL/kg$ over the subsequent 5 hours, totalling $100\ mL/kg$.
Step 3: Plug in the weight of 6 kg. First hour: \[30 \times 6 = 180\ mL\] Next 5 hours: \[70 \times 6 = 420\ mL\]
Step 4: Compare with the options - only one pairs 180 mL initially with 420 mL afterwards, so it is the correct prescription; the rest miscalculate the per-kilogram dosing.\[\boxed{\text{180 ml in 1st hour, then 420 ml over next 5 hours}}\]