Peptization is a:

We first recall the precise scientific meaning of the term peptization. In colloid chemistry, when a freshly prepared precipitate is shaken with a small quantity of a suitable electrolyte (called the peptizing agent), ions from the electrolyte get adsorbed on the surface of the precipitate particles. Because of this adsorption, each particle acquires the same sign of charge. Identical charges repel one another, and, as a consequence, the large aggregates of the precipitate break up into much smaller particles that remain uniformly dispersed in the dispersion medium. Those dispersed particles are of colloidal dimensions, i.e. their diameters lie roughly between $$10^{-7}\,\text{m}$$ and $$10^{-9}\,\text{m}$$, and hence a stable colloidal solution is obtained.

So, in ordinary language, peptization is the process in which we take an existing precipitate and, by the action of a peptizing agent, convert it into a colloidal sol. The key point is that we are going from the precipitated (insoluble, coarse) form to the colloidal (dispersed, stable) form.

Now let us compare this definition with each of the four options given:

Option A states: "Process of converting a colloidal solution into precipitate." This is the exact opposite of what peptization does, so Option A is incorrect.

Option B states: "Process of converting soluble particles to form colloidal solution." Here the starting species are already soluble individual ions or molecules—not a precipitate—so this does not match the definition of peptization. Hence Option B is also incorrect.

Option C states: "Process of bringing colloidal molecule into solution." The phrase "colloidal molecule" is vague, and again the initial state is not a precipitate. Therefore Option C does not accurately represent peptization.

Option D states: "Process of converting precipitate into colloidal solution." This description aligns perfectly with the definition we have just reviewed: a precipitate is transformed into a colloidal sol by means of a peptizing agent. Thus Option D is correct.

Hence, the correct answer is Option D.