Explanation:

- Now that they have passed the stamina test, they can get their scuba diving license.

- This option is correct. "Now that" indicates a cause-and-effect relationship. Since they have passed the test, they qualify for the license.

- Even though they have passed the stamina test, they can get their scuba diving license.

- This option implies an unexpected or contrasting result. It suggests that passing the test wouldn't normally lead to getting the license, which doesn't fit logically.

- Unless they have passed the stamina test, they can get their scuba diving license.

- "Unless" introduces a condition not met. It implies they won’t get the license unless they pass, which reverses the intended meaning.

- When they have passed the stamina test, they can get their scuba diving license.

- This is a reasonable option, suggesting that passing the test is conditional for getting the license, however, it's not the best fit compared to the logically clearer "Now that."