Could the answer be
STACK because the Zen stones are being stacked.
EX. As the element is an isotope of Hydrogen (H), it is literally ex-Hydrogen.
CHANGE. The infinite series changes sign (positive, negative, positive, negative, ...)
Riley Rebus is a calm man
The Zen stones refer to calmness.
"Tritium" sounds like tranquility.
"Equation" sounds like equable.
You will have to think more abstractly than what is literally in the pictures.
Most of the pictures refer to how they sound like (and the prefix has quite nothing to do with Zen stones).
It's not about the stones; It's about the abstract idea that is associated with them.
True. They are stacked.
What does K "look like"? It looks like an X. EK looks like EX. Also, the phrase "Stack Exchange" has EK in it, but backwards (Stack Exchange).
The Stones represent an act often performed by mindful people.
To be mindful means to be aware, and you are aware of more of your surroundings if it is neat and organised. Stacking objects (like books) often creates such a surrounding. "Stacking" also describes an action that of which can be performed.
XXXx Xx XXXXx