The process involved in the formation of interfascicular cambium from fully developed parenchyma cells is known as "dedifferentiation." This is because dedifferentiation refers to the phenomenon where mature, specialized cells revert to a more primitive, less specialized state. In plants, certain mature cells, such as parenchyma cells, can lose their specialized characteristics and revert to meristematic activity. This newly formed meristematic tissue can then differentiate into specialized tissues once again, in this case, forming interfascicular cambium.

In contrast, differentiation (Option A) is the process where cells develop from a less specialized form to a more specialized form. Redifferentiation (Option B) refers to the process where dedifferentiated cells become specialized again, but this is a subsequent step following dedifferentiation. Maturation (Option D) refers to the final stages of cell and tissue specialization, not the regression to a less developed state.

Therefore, the correct answer to the query is:

Option C - Dedifferentiation