The problem with answer choice (A) is that it does not cast doubt on the claim of the proponents of DNA fingerprinting. It says that a large amount of shared material is not included in the DNA fingerprinting procedure. If this is true, then that also means that the patterns in the DNA fingerprinting test are still independent. Since we're looking to weaken the claim, answer choice (A) does not do this for us.
Answer choice (C) does cast doubt on the claim of the proponents of DNA fingerprinting. If it's true, it shows that there are shared characteristics between different subgroups of the whole population, which means that they aren't independent of each other. This wrecks the necessary assumption contained within the argument and weakens it.
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