One person can drink 4.5 litres of water in a day. How many millilitres of water can he drink in 15 days?

The correct option is D 67500 ml
Total number of days $=15$
Litres of water he can drink in 1 day $=4.5l$
Total litres of water that he can drink in 15 days $=15\times 4.5l=67.5l$
Also, 1 l = 1000 ml.
$\Rightarrow$ 67.5 l = 67.5 $\times$ 1000 ml = 67500 ml
Thus, he can drink 67500 millilitres of water in 15 days.