This question can be thought of as a "Strengthen" problem. The conclusion is then the following: The adoption of this policy would be likely to decrease employees' productivity.

The evidence is that the proposed policy would permit each employee to decide when to arrive-from as early as 6 a.m. to as late as 11 a.m.

Which choice strengthens the conclusion? Let's see.

Choice A state that workers must not be interrupted if they're to not have decreased productivity. No problem there -- the conclusion is not touched.
Choice B discusses "other" companies, which is irrelevant to the argument.
Choice C has no effect on the argument.
Choice D is correct. If workers need to interact throughout the entire workday in order to be productive but some will not be in the office at the same time throughout the day, then the argument is strengthened. Productivity will go down.
Choice E is irrelevant. Knowing the duration of the projects -- 1 day, 2, 3 -- does not help us determine whether the new policy will decrease productivity

I'm available if you'd like any follow up.