First Principles Thinking: zero-coupon bond pricing formula
C is correct. For a zero-coupon bond with annual compounding, the price equals the present value of the single future cash flow: $PV = \frac{FV}{(1+r)^N}$. With $PV = 80$, $FV = 100$, and $N = 5$ years (periodicity of 1), the formula is $80 = \frac{100}{(1+r)^5}$, exactly as shown in the CFA Curriculum Example 1.
A is incorrect because $N = 10$ would apply to semiannual compounding over five years, not annual compounding; with periodicity of 1 there are only 5 compounding periods.
B is incorrect because it multiplies rather than discounts, growing the price forward to a future value rather than discounting the face value back to the present.