Solve the Fraction Equation: Finding the Denominator in 18b/? = 3b/a

$$Let's examine the problem:

$\frac{18b}{?}=\frac{3b}{a}$

Remember the fraction reduction operation,

In order for the fraction on the left side to be reducible, all numbers in its denominator must have a common factor. Additionally, we want to reduce the number 18 to obtain the number 3. We also want the term $a$ in the denominator of the fraction on the right side. Note that this term is not found in the expression in the numerator of the fraction on the left side, therefore we will choose the expression:

$6a$

Since:

$18=3\cdot6$

Let's verify that with this choice we obtain the expression on the right side:

$\frac{18b}{?}=\frac{3b}{a} \\ \downarrow\\ \frac{\not{18}b}{\textcolor{red}{\not{6}a}}\stackrel{?}{= }\frac{3b}{a} \\ \downarrow\\ \boxed{\frac{3b}{a}\stackrel{!}{= }\frac{3b}{a} }$

Therefore this choice is indeed correct.

In other words - the correct answer is answer D.