This isn't a necessary assumption for the experts' argument. If anything, this claim weakens the experts' argument, since they are proposing a solution that allows engineers to "quickly return" to their work after typing up outlandish ideas, implying that typing those ideas is not meant to take too much time. Meanwhile, this answer choice suggests that creative engineers actually spend about the same amount of time on doodling as on "serious work."

This isn't a necessary assumption. Even if we negate this claim to state that engineers would sometimes use the notepad for other purposes, as long as they were still typing up outlandish ideas, they would be fulfilling the purpose the experts envision for the program. So the experts' argument wouldn't be any weaker if this claim were negated.

This isn't a necessary assumption. It doesn't need to be true that no engineers working on computers also doodle with paper and pencils. Even if some engineers did so (the negation of this answer choice), it could still be true that most engineers did not, and thus there would still be an overall decrease in useful ideas that could be counteracted by the experts' suggestion.

This is correct. If this claim were false — i.e., if the physical act of working on paper were essential to providing the benefits of doodling — then the experts' suggested program, which is still on a computer, would not effectively counteract the effects caused by a decline in doodling.

This isn't a necessary assumption. Even if only a few of these ideas were incorporated into later projects (the negation of this answer choice), the experts' argument that less doodling would lead to fewer such ideas, and that their program would counteract this decline, could still be true.