(B) says, "Over the last several years, the market for computer games has grown steadily, but Ditalgame's share of that market has shrunk considerably."
The issue with (B) is that this could be caused by the widespread piracy of Ditalgame's games. However, it's also possible that they simply make bad games. Thus, (B) requires an additional assumption in order to make sense. On its own, it's not providing us with any additional evidence to make us think that sales will increase, so it's out.
(C) says, "The copy protection feature causes a copied game to be playable just long enough for most people to come to enjoy it so much that they decide they have to have it."
The second assumption made in the argument is that the people who cannot illegally copy the game will purchase it. (C) strengthens this assumption, as it tells us that the copy protection feature gives people a glimpse of the game just long enough so that they feel like they have to have it, so (C) is the correct answer choice.
Does this make sense? Let us know if you have any other questions!