Explanation:

To find the third proportional to two numbers, say \(a\) and \(b\), you can use the proportion:

$$ \frac{a}{b} = \frac{b}{c} $$

- Here, \(a = 9\) and \(b = 21\).

- Substitute these values into the equation:

$$ \frac{9}{21} = \frac{21}{c} $$

- Cross-multiply to solve for \(c\):

$$ 9 \times c = 21 \times 21 $$

- Therefore,

$$ c = \frac{21 \times 21}{9} = 49 $$

- ? Option 1: 49 is the correct answer.

- Option 2: 12 is not correct based on the calculations.

- Option 3: 30 is also incorrect in this context.

- Option 4: 81 does not fit in the proportion.