In fast food nation, the best reason that the author offers for the fast-food industry's strict regimen for productivity is that
once an order is placed, glowing buttons suggest other menu items.
company operation manuals can be updated more often.
it gives the companies power over their employees.
flashing lights can tell employees what to do.

It gives the companies power over their employees.

Explanation:

In Fast Food Nation, the author describes some of the ways in which fast food restaurants have transformed the industry to their benefit. He tells us that such restaurants profit from hiring students and teenagers as these generally accept a lesser wage and have less confidence in their abilities at work. This lack of confidence and knowledge makes them easier to control. Moreover, the restaurants also exert this power over their employees by maintaining a strict regimen for productivity.