What does it mean when economists say that home buyers are “underwater” on their mortgages?

a.) Buyers owe more on their mortgage than the properties are worth.
b.) Buyers are financially incapable of repaying their mortgages and bankruptcy is inevitable.
c.) Buyers are purchasing homes on flood plains and are highly susceptible to financial losses.
d.) Buyers are paying interest rates substantially higher than current market interest rates, creating interest payments that create financial hardship.