Answer: option D. It does not identify the original cause.
Step-by-step explanation:
Regarding the other options, you can tell:
A. It is supported by large amounts of evidence.
Being it supported by large amounts of evidence, this is not an argument against but pro the theory. So, of course, you cannot use it against the theroy.
B. It is just an educated guess.
This is not true. As stated in the previous choice, it is supported by large amounts of evidence. So, you cannot seriously tell that it is an educated guess.
C. It cannot be tested directly.
The very fact that it cannot be tested directly is not sufficient argument, as there are a lot of evidence. You need to refute the actual evidence but not tell that it has not been tested directly if the indirect tests provide good evidence.
The point D, that it does not identify the original cause, is a valid argument, as a theory must provide explanations and in this respect the theory just tells that the singularity does not permit go a single instant before the big bang moment preventing to know the causes. This is, the theory is not providing explanation (the cause) for an important fact.