The ability of a substance to exist in two or more crystalline forms knows as:

Options

A. isomerism
B. polymorphism
C. isomorphism
D. amorphism

Explanation

To solve the question regarding the ability of a substance to exist in two or more crystalline forms, we will analyze the provided options step by step.

### Step 1: Understand the Options
The options provided are:
1. Isomerism
2. Polymorphism
3. Isomorphism
4. Amorphism

### Step 2: Define Each Term
- **Isomerism**: This refers to the existence of molecules that have the same number of the same kind of atoms (same molecular formula) but differ in their physical and chemical properties. This does not pertain to crystalline forms.

- **Polymorphism**: This is the phenomenon where a substance can exist in more than one crystalline form. For example, calcium carbonate can exist as calcite and aragonite. This directly relates to the question.

- **Isomorphism**: This refers to two or more crystals that have the same composition and the same crystalline form. This is not about different crystalline forms.

- **Amorphism**: This describes a state where the constituent particles do not possess a three-dimensional arrangement, which means it does not relate to crystalline forms.

### Step 3: Identify the Correct Answer
From the definitions:
- Isomerism is incorrect as it pertains to molecular structures, not crystalline forms.
- Polymorphism is correct as it specifically refers to the ability of a substance to exist in multiple crystalline forms.
- Isomorphism is incorrect as it refers to the same crystalline form.
- Amorphism is also incorrect as it refers to a lack of crystalline structure.

Thus, the correct answer is **Polymorphism**.

### Final Answer
The ability of a substance to exist in two or more crystalline forms is known as **Polymorphism**.

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