I might agree with you to a certain extent.
the source is unknown (well, it is from 1000 series, but is it a seflmade question or it came from GMAT-paper tests, or any other source i don't know).
A good question does not require additional assumptions to arrive to correct answer choice.
The argument says:
premise 1: "the more Colombian beans in a blend of coffee, the better the blend".
It means that best blend is achieved when a coffee maker uses 100% of Columbian beans.
premise 2: "no company purchases more Colombian beans than Kreemo Coffee. Inc"
Conclusion "Kreemo’s coffee is best blended
coffee " (this is why E is wrong).
So the K's coffee is best because they buy a lot of C beans.
To weaken a question one must weaken the conclusion. But, the correct answers do not have to absolutely, without a doubt, in all circumstances weaken the conclusion. They only have to open up the possibility
that the conclusion is not valid. The correct answer on a weaken question will typically accomplish this by introducing a new piece of information that calls into question an assumption made by the author.
The possible assumptions made by author are:
1. KC does not resell Columbian beans,
2. KC uses all Columbian beans in production of coffee
3. None of competitors puts more Columbian beans in coffee than KC does.
4. Quantity of Columbian beans and the coffee sold as cans perfectly correlates.
5. beans are used in production of coffee.
You are right saying that:
it overlooks the possibility that the coffee blend in the can may not even contain the purchased Columbian beans
your statement attacks the 2nd assumption.
I definitively agree with this statement:
" is not reselling the columbian beans at a higher price and used some other bean for the coffee blend"
it weakens the 1st assumption.
this is are both very valid weakening arguments, but in test-makers view these are too easy to spot.
Thus probably they decided to use the argument that weakens the 3rd assumption:
"C) Kreemo sells more coffee than does any other company "
but this answer is too stretched. Why?!
Because it requires additional assumptions, such as "all coffee beans that KC buys, it uses later in production", "purchases are equal to sales", "no columbian beans are stored in stock", etc...
But C opens up a possibility that high volumes does not necessarily mean high % or in our case higher quality
In my example above you may change the figure 20 in 400 resulting in 40% instead of 2% concentration of Columbian beans in a cup of coffee. This strenghtens the argument.
But also the total volume of beans a company purchases leaves a gap in possibility to intrepret differently the given information.
If the argument said that KC sells less coffee than any other company and buys more columbian beans than any other company, it would be much difficult to weaken such argument. (becuase it would result in higher concentration of coffee beans).
Hope it helps.
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