The correct answer is D. about.
Ai Ling is worried about the test because she thinks that she will fail the test. This means that she is concerned about the test and what might happen if she does not do well. She may be worried about her grades, her future, or even her self-esteem.
The other options are not as accurate. Option A, of, is not correct because Ai Ling is not afraid of the test. She is not worried that the test will hurt her or that she will be in danger. Option B, on, is not correct because Ai Ling is not working on the test. She is not trying to complete it or make it better. Option C, to, is not correct because Ai Ling is not going to the test. She is not planning to attend it or take it.
In conclusion, the correct answer is D. about. Ai Ling is worried about the test because she thinks that she will fail the test.