According to a recent study of sexual behavior in the U.S., over half of the couples that were married and engaged at the time of the study's survey were living together before making a final commitment to each other.

couples that were married and engaged at the time of the study's survey were living
couples that were engaged or married at the time of the study's survey had lived
couples who were engaged or married at the time when the study's survey occurred were living
married and engaged couples at the time of the study's survey had lived
married or engaged couples at the time of the study's survey were living

3:2 - X:B and D
I chose D because of "that"

I know that "that" can modify committee, team....But, I was skeptic about the usage on the GMAT....

Any thoughts ?

OA - B obviously...