Let's volume of mixture \(A\) be \(200 \ ml\), indicating that the quantity of cocoa in the mixture is \(120 \ m\)l and the quantity of sugar is \(80 \ ml\).
Similarly, considering a mixture volume of \(300\ ml\), it implies that the quantity of coffee and sugar in the mixture is \(210\ ml\) and \(90 \ ml\) respectively.
Now, when we combine mixture \(A\) and \(B\) in the ratio of \(2:3\) (meaning if \(200\ ml\) of mixture \(A\), then \(300\ ml\) of mixture \(B\)), the volume of the resulting mixture \( C= (200 + 300) = 500 \ ml\),
and the quantity of sugar \(= (90 + 80) = 170\ ml.\)
Then, combining mixture \(C\) with an equal amount of milk to make a drink, the final mixture's quantity \(= (500 + 500) = 1000 \ ml.\)
The quantity of sugar in the final mixture remains \(170 \ ml\).
Therefore, the percentage of sugar in the final mixture is \(17\%\).
So, the correct option is \((C): 17\).