Answer: Pure expectations
Explanation: This theory suggest that the current forward rates depicts the interest rates that are going to be in the future. Using such a theory the investor makes investment based only on the expectations of future interest rates. This is generally seen in the bond market.
In the given case, Yvonne told that the reason behind the upward slope is due to the expectations of the investors that interest rates will increase in the future.
Thus, we can conclude that her observation depicts evidence of pure expectation theory.