UTD says previous abruption is the strongest risk factor for placental abruption, not the most common. It also says that placental abruption has an incidence of 3-10 per 1000 live births, which is significantly lower than the incidence of hypertension in pregnancy (2-17% of pregnancies, per UTD). Amboss reports hypertension as the most common risk factor for abruption.
I would be curious on the exact wording and source of the Rosh question; you might reach out to their support person and ask about it if sources differ.