What term was coined by Weber's student Robert Michels to describe the tendency in organizations toward the centralization of power, which can be detrimental to democratic processes?

The correct answer to the following question will be "Iron law of oligarchy".

Explanation:

A statute or theory of power, which is unquestionable and implacable iron rules of traditional requirement. Throughout the establishment, there seems to be a quite tiny minority of people who make decisions, even though the power is officially entrusted to the affiliation body at vast. This legislation may be extremely damaging to democratic institutions.

Therefore, it's the right answer.