Explanation:

To find the fourth proportional to 7, 13, and 21, use the concept of proportionality:

- Set up the relationship: 7:13 = 21:x

- Solve for x by cross-multiplying: 7x = 13 * 21

- Calculate: 7x = 273

- Divide both sides by 7: x = 39

Reviewing your options:

- Option 1: 26 – Result from different proportion

- Option 2: 39 – This solves the equation

- Option 3: 14 – Not a result of the equation

- Option 4: 21 – Is the third given number

Thus, the correct answer is:

Option 2: 39