Step 1: Polyuria means an abnormally large volume of urine production over a day.
Step 2: By the standard weight-based criterion, polyuria is defined as urine output greater than 50 ml/kg/day. So the answer is 50 ml/kg (option c).
Step 3: In adults this roughly corresponds to more than about 3 litres of urine per day, which is the commonly quoted clinical threshold.
Step 4: The other values - 70, 60 and 40 ml/kg/day - do not match the accepted cut-off, so options a, b and d are incorrect.