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

$$Let's examine the problem:

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

Now let's think logically, and remember the fraction reduction operation,

For the fraction on the left side to be reducible, we want all numbers in its denominator to have a common factor, additionally, we want to reduce the number 18 to get the number 3, and we also want to get in the denominator of the fraction on the right side the term $a$, 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 indeed get 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.