Due to ongoing instability in the real estate market, many people
that might at one time have bought homes as young adults are choosing to forgo such purchases in favor of living in rented accommodations.
once - implies some time in the past, whereas at one time - implies “in or during a known but unspecified past period”
(A) that might at one time have bought homes as young adults
Incorrect – who is preferred over that to refer to people. Meaning issue – at one time bought homes implies that we are not sure if they still own the house.
(B) that as young adults might once have bought homes
incorrect - who is preferred over that to refer to people
(C) who might once have bought homes in young adulthood
Correct - clear and concise. In young adulthood is adverbial phrase that modifies have bought. Hence, who might once have bought homes in young adulthood clearly suggests that many people might have bought homes when young.
(D) who in young adulthood might have at one time bought homes
Incorrect - who might have at one time bought homes implies that we are not sure if they still own the house.
(E) who, when they were young adults, might at one time have bought homes
Incorrect - option E is not logical. It seems to suggest that people bought homes at one time when they were young adults and maybe at other times they may have not bought homes.
Answer C