Understanding the argument -
We must strengthen the conclusion that the hourly wages must be higher at Yorco (Y) than at Zortech (Z). The facts given in the argument are
1. They pay their employees per hour
2. Both employ the same number of hourly wage workers.
Prethinking - In such a situation, the higher cost can occur mainly because of two reasons
1. Y's employees do a ton of overtime.
2. Work done by Y requires niche skills that are expensive to find, thus pushing the cost higher.
Option Elimination -
(A) Zortech spends a higher total sum per year than Yorco does to provide its hourly wage workers with benefits other than wages - Benefits other than wages are out of scope.
(B) the work performed by hourly wage workers at Zortech does not require a significantly higher level of skill than the work performed by hourly wage workers at Yorco does - It says Z's workers do not require a significantly higher level of skills than Y's workers. The trap is that "not significantly higher" does not mean it is lower. It can also mean it is somewhat higher but not significantly higher. In that case, it's the opposite of what we seek. So, at best, this option is a weakener.
(C) the proportion of all company employees who are hourly wage workers is significantly greater at Yorco than it is at Zortech - If we read the sentence carefully - it says the proportion (of all company employees who are hourly wage workers) is significantly higher than it (the proportion of all company employees who are hourly wage workers) at Zortech. The proportion is a percentage, i.e., per 100. So it's comparing the "employees who are hourly wage workers" to what? Are there any other kinds of workers as well? The premise states that two corporations employ "large numbers of" full-time workers paid by the hour. So it means that a large number is paid per hour, but a large doest means 100%. It means there is another category of employees which is not given in the argument. So, comparing proportion to another category of employees is not in the scope of our discussion here. We are mainly concerned about such workers (full-time employees who are paid hourly); the overall wage spent for such workers for company Y is more than Z. So, at best, this choice is out of scope.
(D) overtime work, which is paid at a substantially higher rate than work done during the regular work week, is rare at both Yorco and Zortech -perfect. In line with our pre-thinking.
(E) the highest hourly wages paid at Yorco are higher than the highest hourly wages paid at Zortech - We are concerned about the average wage. Distortion.