<u>Given</u>:
The given triangle is a similar triangle.
The length of the hypotenuse is 18 units.
The length of the leg is a.
The length of the part of the hypotenuse is 16 units.
We need to determine the proportion used to find the value of a.
<u>Proportion to find the value of a:</u>
We shall find the proportion to determine the value of a using the geometric mean leg rule.
Applying the leg rule, we have;
Substituting the values of hypotenuse, leg and part, we get;
Thus, the proportion used to find the value of a is
Hence, Option D is the correct answer.