The third proportional to 9 and 18 is

1. 21
2. 24
3. 27
4. 36

Given:

Numbers = 9 and 18

Let the third proportional be x.

9 : 18 : : 18 : x

Product of Means = Product of Extremes

9 × x = 18 × 18

⇒ x = $\dfrac{18 \times 18}{9}$

⇒ x = 2 x 18

⇒ x = 36

Hence, option 4 is the correct option.