Which one of these is NOT a type of bond strategy?

The correct answer identifies "Put-to-call" as not a type of bond strategy. This term typically relates to options rather than fixed-income securities. A put option gives the holder the right to sell a specified amount of an asset at a predetermined price before a certain date, while a call option allows purchasing. These concepts are more associated with stock and derivatives trading rather than managing bond investments.

In contrast, the other choices represent established bond strategies. "Income preserving" involves methods focused on maintaining or enhancing income through careful selection and management of bonds. "Zero-coupon" bonds are a specific type of bond that is sold at a discount and does not pay periodic interest, appealing for their ability to provide a lump sum at maturity. "Bond laddering" is a strategy that involves purchasing bonds with varying maturities to manage interest rate risk and liquidity needs effectively. Each of these strategies serves specific investment goals, showcasing their relevance in the realm of bond investing.