Hey All,
I got asked to explain two things on this question.
1) Why is E wrong. E is wrong because it gets the meaning scrambled. Take a look:
couples in which both of the spouses working outside the home become more common
The question is what is becoming more common. In this one, "become more common" has the subject "both of the spouses working outside the home." But that's not what we want. We want THE FACT that both spouses work outside of the home to be becoming more common.
Which brings me to the second questions: 2) What is the "it" standing in for?
This is a special "it" that gets used in the subject of object position to refer to a specific or situational subject or object later in the sentence. For example: It is hot out today. It's likely you'll come over later.
The "it" here is exactly what we want to be more common; the situation of two working spouses in households.
Does that make sense?
-t