Final answer:
Wood is the fuel that is most susceptible to changes in relative humidity.
Step-by-step explanation:
The fuel that is most susceptible to changes in relative humidity is wood. Wood is a renewable biomass fuel that contains moisture. When the relative humidity of the surrounding air increases, wood absorbs moisture from the air, causing its moisture content to increase. On the other hand, when the relative humidity decreases, wood releases moisture to the air, causing its moisture content to decrease.