Zero-coupon bonds are a type of bond that does not pay interest (a coupon) during its life. Instead, investors buy zero-coupon bonds at a deep discount from their face value, which is the amount a bond will be worth when it 'matures' or comes due. When a zero-coupon bond matures, the investor will receive one lump sum equal to the initial investment plus interest that has accrued. This makes zero-coupon bonds unique among most types of bonds on the market, which typically pay out interest semiannually. The discount in price effectively represents the 'interest' the bond pays to investors. Therefore, by definition, zero-coupon bonds are always issued at a discount price.

Choice B is incorrect. Straight-coupon bonds are not always issued at a discount price. They pay periodic interest payments and the face value at maturity, and can be issued at par, premium or discount depending on the market interest rates.

Choice C is incorrect. Floating rate bonds have variable interest rates that are tied to a benchmark such as LIBOR or treasury bills. These bonds are not necessarily issued at a discount because their coupon rate adjusts with changes in market rates.

Choice D is incorrect. Premium bonds are actually sold for more than their face value (at a premium), hence they cannot be the type of bond that is invariably issued at a discount price.