Explanation:

To determine the missing number in the series, follow these steps:

- Identify the pattern: Look at the differences between consecutive numbers.

- 318 - 209 = 109

- 427 - 318 = 109

- 536 - 427 = 109

- The difference between each consecutive number seems to be 109.

- Use this pattern to find the missing number:

- 536 + 109 = 645

- Validate the series with the final given number:

- 645 + 109 = 754

- Check options:

- Option 1: 655

- Option 2: 645

- Option 3: 664

- Option 4: 635

The missing number is 645, making Option 2 the correct choice.