A recently published report indicates that the salaries of teachers

As described in this post, the use of "to" ("to almost $24,000") illogically implies a range of values ("... by an average of $8,000... to $24,000"). That makes it sound as though the amount of lag is, on average, between $8,000 and $24,000. But that's not the intended meaning.

We are told "that the salaries of teachers continue to lag far behind [the salaries] of other college-educated professionals". By how much do the salaries of teachers lag behind the salaries of other college-educated professionals? 1) By an average of nearly $8,000 a year at the start of their careers and 2) by almost $24,000 a year by the time they reach the age of 50.

At time 1, the salaries of teachers lag behind BY some amount (an average of nearly $8,000 a year). At time 2, the salaries of teachers lag behind BY some other amount (almost $24,000 a year). 1) and 2) both answer the question, "By how much do the salaries of teachers lag behind?". So each needs to start with "by", as in choice (E).

There also might be another issue with (D), if in fact there has been a typo ("...by an average of UP TO nearly $8,000..."). If anyone encounters this question on a practice test, please confirm the exact wording of choice (D) and/or post a screeenshot.

As we've mentioned in other threads, the "rules" governing comma usage can be subtle and subjective, so you don't want to be too rigid when debating whether a comma is appropriate. You won't run across many GMAT questions that explicitly test whether the presence or absence of a comma is correct, so it's best to find more concrete reasons to eliminate answer choices. For more on punctuation on the GMAT, check out this video.