Convexity is a measure of the curvature in the relationship between bond prices and bond yields that demonstrates how the duration of a bond changes as the interest rate changes. This measure is used to assess the sensitivity of the price of a bond to changes in interest rates. Convexity is particularly useful when there are larger changes in interest rates, as it provides a more accurate estimate of bond price changes than duration alone. Convexity takes into account the effects of changes in the yield curve on bond prices, which can be significant when the changes in the yield curve are large. Therefore, convexity is the most appropriate measure to use when estimating changes in bond prices due to larger changes in the yield curve.

Choice A is incorrect. Duration is a measure of the sensitivity of the price of a bond to changes in interest rates. However, it assumes that this relationship is linear and does not account for substantial changes in the yield curve.

Choice C is incorrect. Modified duration, like duration, measures the price sensitivity of a bond to interest rate changes but it also assumes a linear relationship between bond prices and yields. It fails to capture the effects of larger shifts in the yield curve.

Choice D is incorrect. Concavity isn't used as a metric in bond investing for predicting alterations in bond prices due to yield curve fluctuations. It's rather an attribute related to convexity which describes how the change in yield affects convexity itself.