Author's reasoning is based on the assumption that "Simulated Notepad" will have the same impact as "paper & pen"; the option {D} clearly states that

sana.noor wrote:Studies show that the most creative engineers get their best and most useful ideas only after doodling and jotting down what turn out to be outlandish ideas. Now that many engineers do their work with computers instead of on paper, however, doodling is becoming much less common, and some experts fear that the result will be fewer creative and useful engineering ideas. These experts argue that this undesirable consequence would be avoided if computer programs for engineering work included simulated notepads that would allow engineers to suspend their "serious" work on the computer, type up outlandish ideas, and then quickly return to their original work.

Which one of the following is an assumption on which the experts' reasoning depends?

(A) Most creative engineers who work with paper and pencil spend about as much time doodling as they spend on what they consider serious work.
(B) Simulated notepads would not be used by engineers for any purpose other than typing up outlandish ideas.
(C) No engineers who work with computers keep paper and pencil near their computers in order to doodle and jot down ideas.
(D) The physical act of working on paper is not essential in providing engineers with the benefits that can be gained by doodling.
(E) Most of the outlandish ideas engineers jot down while doodling are later incorporated into projects that have practical applications.

Conclusion:
... this undesirable consequence (doodling is becoming much less common --> fewer creative and useful engineering ideas) would be avoided if computer programs for engineering work included simulated notepads that would allow engineers to suspend their "serious" work on the computer, type up outlandish ideas, and then quickly return to their original work.

We need to find the assumption made by the experts; answer choices which could be valid assumptions are C/D, others are irrelevant.

for finding out the correct answer use the NEGATION Technique (teh negated sentence should DESTROY the argument completely)

(C) Negated: ALL engineers who work with computers keep paper and pencil near their computers in order to doodle and jot down ideas.
According to this negation, the engineers have an option to doodle their ideas on a physical notepad while working with a computer, however the argument states that the act of doodling itself has reduced drastically. Engineers may not (or may) use the physical notepad at all for doodling their ideas. This negation actually doesn't destroy the argument completely.

(D) Negated: The physical act of working on paper is essential in providing engineers with the benefits that can be gained by doodling.
Many engineers now work with computers --> experts feel that a simulated notepad provided by the computer program would help avoid the problem of "fewer creative and useful engineering ideas". Our negated sentence says that the act of working on a paper provides these "good" ideas and thus their assumption that a simulated notepad would solve all the problems is DESTROYED Completely.

[spoiler]Answer : D[/spoiler]