1:3
4:9
1:2
5:9
Initial state: 16 units of coffee, 9 units of cocoa. Total units = 25.
A mixture of x units is removed and replaced with cocoa.
After removal, remaining coffee = (25 - x) units.
According to the problem statement:
\((25-x)(1-\frac {x}{25}) = 16\)
\((25-x)\frac {(25-x)}{25} = 16\)
\((25-x)^2 = 25 \times 16\)
\((25-x)^2 = 400\)
\((25-x) = \sqrt {400}\)
\(25-x= 20\)
\(x = 5\)
Final mixture P: 5 units of cocoa.
Initial mixture Q: 9 units of cocoa.
The required ratio is 5:9.
Therefore, the correct option is (D): 5:9.