The existence of a substance in more than one solid modification is known as:
(a) Isomorphism
(b) Polymorphism
(c) Amorphism
(d) Allotropy

Hint: The organic compound glycine is exhibiting the phenomenon of polymorphism. It has two polymorphs i.e., hexagonal, and monoclinic crystals.

Complete Step by Step Solution:
Isomorphism
- When two or more crystalline solids possess the same chemical composition along with similar crystalline form. This behaviour is called an isomorphism. For example., \[N{a_3}P{O_4}\] and \[N{a_3}As{O_4}\].

Polymorphism
- The solid compounds exist in two different crystalline forms.
- The polymorph has different physical properties and sometimes chemical properties such as., melting point, colour, hardness, solubility, dissolution rate, as well as variance in chemical reactivity.
- Isomerism is a type of Polymorphism.

Types of polymorphism
- On the basis of the stability of crystalline form, polymorphism can be classified into two types.

Mono-tropic polymorphism
- In mono-tropic polymorphism, only one polymorph is stable for all the acceptable temperatures. E.g., metolazone exhibits such type of polymorphism.

Enantiotropic Polymorphism
- In the enantiotropic system of polymorphism, there are different polymorphs, and each polymorph is stable under a specific range of temperatures. Thus, one polymorph is stable at a low temperature, another can be stable at a high temperature, and so on. E.g., acetazolamide and carbamazepine exhibit this type of polymorphism.
- Polymorphism is very useful in pharmaceuticals to enhance the bioavailability and solubility of drugs.

Amorphism
- The amorphous solid is also known as a super-cool liquid.
- In the structure of an amorphous solid, the constituent atoms do not follow the regular three-dimensional arrangement i.e., they exhibit short-range order.
- They do not have sharp or fixed melting points i.e.; they have a wide range of melting temperatures.
- They do not have the ability to diffract x-rays.
- They do not possess a plane, axis, and centre of symmetry.
- The amorphous solids are also considered isotropic in nature because their physical properties are the same in all directions.

Allotropy
- When the same compound exists in different forms, this phenomenon is known as allotropy and the forms of compounds are called allotropes. The allotropic can differ due to the different arrangement of constituent particles. For example, carbon, phosphorus, and Sulphur.
- Carbon is found in various allotropic forms.
- A crystalline from Diamond, Graphite, and Fullerenes.
- Amorphous from: Coal, charcoal.
From the above conclusion indicates that the answer (b) will be correct.

Note: The polymorphs have different physical properties such as solubility, melting point, hygroscopicity., etc., and the same chemical properties.