What word means thoroughly soaked or saturated?

The term that signifies being thoroughly soaked or saturated is "sodden." This word conveys a strong sense of saturation, typically relating to something that is excessively wet or soaked through, like soil after a heavy rain or a sponge filled with water. It emphasizes the condition of being moist to the point where it may become heavy or unable to hold any more liquid.

In contrast, the other options do not carry this meaning. "Serene" refers to a state of calmness or tranquility, "staid" describes someone who is respectable and unadventurous, often associated with a sober demeanor, and "spurn" means to reject something disdainfully. Therefore, “sodden” is the correct choice for describing something that is completely soaked.