Kraft temperature is the temperature:

We begin by recalling some basic facts about detergents (also called surfactants). Such molecules possess two distinct parts: a long non-polar hydrocarbon chain (the tail) and an ionic or polar head group. Because of this dual nature, when they are dispersed in water, two key phenomena are observed:

1. At very low concentration each detergent molecule stays isolated in the solution.
2. When the concentration reaches a certain minimum value, called the critical micelle concentration (abbreviated as $$\text{CMC}$$), the molecules spontaneously aggregate so that the hydrophobic tails hide inside and the hydrophilic heads remain in contact with water. These aggregates are known as micelles.

However, concentration is not the only factor that governs micelle formation. Temperature also plays an essential role. For many ionic detergents, even if the concentration is raised to the CMC, micelles will not form unless the solution is warmed beyond a particular characteristic temperature. This special temperature is termed the Kraft temperature, symbolised as $$T_K$$.

By definition, the Kraft temperature is that temperature above which the solubility of the detergent in water increases abruptly and micelles start forming, and below which the detergent remains mostly as separate monomer molecules or even as a crystalline solid with negligible micelle formation.

Thus, if we phrase the definition using the options given:

“Kraft temperature is the temperature above which the formation of micelles takes place.”

This wording exactly matches Option D in the question.

Hence, the correct answer is Option D.