Which term describes the action of causing something to expand or disseminate?

The term "propagate" refers to the action of causing something to expand, spread, or disseminate, which aligns perfectly with the context of the question. In various contexts, such as biology or communication, to propagate means to breed or multiply organisms or to spread information, ideas, or traits from one place or person to another. This term encompasses the sense of growth and dispersal, making it the most suitable choice.

While the other options present different meanings—pruning relates to cutting back in gardening or reducing something, rectifying pertains to correcting or fixing an issue, and saturating refers to soaking thoroughly or filling to capacity—they do not convey the notion of expansion or dissemination. Therefore, "propagate" effectively captures the essence of causing something to grow or spread.