The coupon or interest on a bond is calculated on the par value of the bond. The par value, also known as the face value, is the amount that the bond issuer originally receives from the bondholder and promises to repay upon the bond's maturity. The coupon rate is expressed as a percentage of this par value. For instance, if a bond has a par value of $1,000 and a coupon rate of 5%, the annual coupon payments would be $50. This is because the coupon rate is applied to the par value of the bond to determine the annual coupon payments.

Choice A is incorrect. The coupon or interest of a bond is not calculated on the price of the bond. The price of a bond can fluctuate due to market conditions, but this does not affect the calculation of the coupon or interest which is fixed at issuance based on par value.

Choice B is incorrect. While amortized value can be used in certain calculations related to bonds, it's not used for calculating coupons or interest payments. Amortized cost accounting involves gradually writing off an intangible asset over a period of time, which doesn't apply here.

Choice D is incorrect. Although it might seem that coupons are always fixed in dollar terms because they are often quoted as such, this statement isn't universally true for all types of bonds. For example, floating rate notes have variable coupons tied to reference rates like LIBOR or Euribor and hence their dollar amount isn't always fixed.