Necessary, because if it were not true — if computers CAN currently be made faster without their CPU chips being made smaller — then the fact we can’t make CPU chips smaller without decreasing their sophistication would no longer constitute a reason we can’t make computers faster. (A) is necessary in order for the author to believe that the constraint of not being able to make CPU chips smaller without decreasing their sophistication is something that limits our ability to make computers faster.

Not necessary, the premise tells us that we can’t make chips significant smaller “without decreasing their sophistication.” But that leaves open the possibility that if we DO decrease sophistication, then we might be able to make CPU chips smaller.

Not necessary, because although the author must assume that making a chip smaller without decreasing sophistication is something required to make the computer faster, this doesn’t imply that as both size and sophistication go down, the chip speed will become SLOWER. Nothing in the argument requires anything to become slower.

Not necessary, because the reasoning of the argument doesn’t depend on what computer manufacturers believe.

Not necessary, because although we know from the context (in the first half) that decreasing size without decreasing sophistication will make a chip faster, that doesn’t imply that INCREASING sophistication while maintaining size will also increase speed.