The amorphous form of silica is:

We begin by recalling that silica, denoted by the chemical formula $$\text{SiO}_2$$, can exist in many structural varieties. These varieties are broadly classified into two groups on the basis of the long-range order in the arrangement of $$\text{SiO}_4^{4-}$$ tetrahedra:

• Crystalline forms, where the tetrahedra are arranged in a regular, repeating three-dimensional pattern.
• Amorphous forms, where no such long-range periodicity is present and the tetrahedra are linked in a random network.

Now, let us examine each option and decide whether it is crystalline or amorphous.

We have quartz. Quartz is the most common naturally occurring crystal of $$\text{SiO}_2$$; its atoms are packed in a highly ordered trigonal lattice. Hence quartz is crystalline.

Next, we have kieselguhr (also called diatomaceous earth). It is formed from the microscopic skeletons of diatoms and possesses a porous, powdery structure with no long-range order. Therefore kieselguhr is amorphous.

Then comes tridymite. Tridymite is another polymorph of $$\text{SiO}_2$$ that appears at higher temperatures than quartz and has a distinct, ordered crystal lattice. Thus tridymite is crystalline.

Finally, we have cristobalite. Cristobalite is yet another high-temperature polymorph of silica with its own well-defined crystal system, so cristobalite is crystalline as well.

Out of the four names considered, only kieselguhr lacks a regular, periodic crystal lattice. All the remaining three are crystalline polymorphs of silica.

So the form of silica that is amorphous is kieselguhr, which corresponds to Option 2 in the given list.

Hence, the correct answer is Option 2.