Explanation:

To find the fourth proportional of three numbers a, b, and c, you use the formula: fourth proportional = (b * c) / a.

- Given numbers: 18, 27, 68.

- Formula:

$$

\text{Fourth Proportional} = \frac{(27 \times 68)}{18}

$$

- Calculation:

- Multiply 27 by 68 to get 1836.

- Divide 1836 by 18 to get 102.

- Options explained:

- Option 1: 136 - Doesn't fit the calculation.

- Option 2: 170 - Not the result of the calculation.

- Option 3: 204 - Wrong result from the formula.

- Option 4: 102 - Matches the calculation.

- Correct Answer: Option 4: 102