I'm with C
D can be refuted because the conclusion says that "the majority
of those who test positive will be people who have used cocaine" leaves some leeway that (D) "some cocaine users do not test positive" can still be true. Hence, saying that (D) ignores that some... is wrong.
Category 1: 5% of people who do not use cocaine will test positive for it
Category 2: 99% of people who use cocaine will test positive for it
B is not necessarily the flaw in the reasoning. If we actually take 1 person who tests positive, there is just an equal chance that he will be part of category 1 as category 2. The only way for us to know is to find out what proportion of the whole population belongs to category 1 and what proportion belongs to category 2.
For instance, if there is 1000 people who used cocaine and 99,000 who did not. Then, we know that 1% of the population used cocaine(and it usually is the case that there are less people of a whole population who use drugs than people who do not use it. Otherwise, we would be living in mad city). Also, we will know that .99% of the people who tested positive will actually have used cocaine. Conversely, we will also know that .05*99% = 4.95% of people who tested positive did not use cocaine. Therefore, someone who tested positive has more chance (4.95%) of falling within the people who did NOT use cocaine than falling within the people(.99%) who DID use cocaine. Therefore, the conclusion that "the vast majority of those who test positive will be people who have used cocaine" is totally wrong. C must therefore be the error in the reasoning. Was that right?