Final Answer:
Option C, 'The data used in the argument is incomplete,' least strengthens the argument above, thus the correct option is C.
Explanation:
Strengthening an argument means adding evidence or facts which support the argument and make it more valid. The given options are A) experts in the field express agreement with the argument, B) the argument is consistent with established scientific principles, C) the data used in the argument is incomplete and D) the argument has been widely discussed in academic circles. Option C, 'the data used in the argument is incomplete,' least strengthens the argument. This is because incomplete data does not add any value to the argument. It does not prove or disprove the argument, and hence does not add to the strength of the argument.
On the other hand, option A, 'experts in the field express agreement with the argument,' strengthens the argument as it adds credibility to it. This is because having experts agree with the argument implies that the argument is valid and holds true. Similarly, option B, 'the argument is consistent with established scientific principles,' also strengthens the argument as it implies that the argument is based on scientific principles and therefore is true. Option D, 'the argument has been widely discussed in academic circles,' also strengthens the argument as it implies that the argument has been discussed and accepted by a wide audience, implying that it is valid.
Therefore, it can be concluded that option C, 'the data used in the argument is incomplete,' least strengthens the argument above.