A person is more likely to become disabled as that person ages.
Curious Fact
Given that one is more likely to be disabled as one ages, why is it that 4% of people aged 55-64 receive disability benefit payments, 2% of 65-74 year olds receive it, and only 1% of people 75 and up receive it?
In essence, given that you're more likely to be disabled, the older you are, why is there a smaller % of super old people getting disability benefits than there are of younger old people?
Explanation
Not as many jobs offered such a disability benefit until recent years.
Evaluate
We should sit with this explanation a second if it doesn't immediately seem clear how it's explaining the data.
The author is suggesting that if all jobs offered disability benefits, then the older a segment of the population is, the higher a percent of them would be receiving disability benefits.
But since most jobs weren't offering disability benefits until recently, the people 75 and up (who presumably retired 10 years ago) might be disabled but they wouldn't receive disability benefits from their former employer because back when they were working, employers didn't offer disability benefits.
Ultimately, though, we really don't need to fixate too much on this explanation, since the question stem is seeking some additional explanation. In order for us to show that this explanation is at best incomplete, we can show that something else is also having an effect on why a smaller % of the oldest demographic gets disability benefits.
Goal
The correct answer may surprise us and provide a reason to think that the author's explanation cannot plausibly account for all of this 4% / 2% / 1% gradient.
But since the question stem asks us to show that this explanation is incomplete, it seems to be leading us to the idea that .
So we need something else, besides the idea that more jobs these days offer disability benefits, that could explain why a smaller % of 75 year olds receive benefits than do 65 year olds, and why a smaller % of 65 year olds do than 55 year olds.
(Maybe something like, )