this is the conclusion: Clearly, making our jigsaw puzzles easier to complete is the best way to increase our sales.
the plan is to make puzzles easier, and this in result will increase sales.
let's take answer choice A:
People rarely do puzzles a second time and may give away puzzles after completing them
if they do not make puzzles second time -
will this make them buy new ones? we don't know; we are not sure whether this will lead to an increase in sales.
answer choice C:
One of the main reasons that people purchase jigsaw puzzles is for the challenge
if people do purchase puzzles
just because they like hard ones, will they purchase new ones? No. This clearly weakens the conclusion of the argument that the company's sales will increase, making easier puzzles.
we have to choose the answer choice that simply destroys the conclusion, and C is the answer choice that does that.