Isomerism:
Isomerism is the phenomenon where two or more compounds have the same molecular formula but different structural arrangements or spatial orientations, resulting in different chemical and physical properties. Isomers can be classified into several types based on the nature of differences in their structures.

Types of Isomerism:
There are several types of isomerism, including:

1. Structural Isomerism:
- Also known as constitutional isomerism, this type of isomerism arises from differences in the connectivity or arrangement of atoms within a molecule.
- It can be further classified into chain isomerism, functional group isomerism, tautomeric isomerism, and ring-chain isomerism.

2. Stereoisomerism:
- Stereoisomers have the same connectivity of atoms but differ in the arrangement of atoms in space.
- This type of isomerism can be further divided into geometric (cis-trans) isomerism and optical isomerism.

3. Metamerism:
- Metamerism is a type of isomerism where two or more compounds have the same molecular formula and connectivity but differ in the nature or position of a functional group.
- It is commonly observed in compounds containing heteroatoms such as oxygen, nitrogen, or sulfur, where the functional group is attached to different carbon atoms.

4. Tautomeric Isomerism:
- Tautomeric isomerism is a special type of structural isomerism where the interconversion between isomers involves a shift of a hydrogen atom along with rearrangement of bonds.
- It is commonly observed in compounds containing a carbonyl group (C=O) or an enol group (C=C-OH).

Answer to the Question:
The phenomenon described in the question, where two or more compounds possess the same molecular formula but different properties, is known as isomerism. Isomerism occurs due to differences in the structural arrangement or spatial orientation of atoms within the molecules. Therefore, option 'D', which represents isomerism, is the correct answer.

Isomers bcz having many different structural formulas but Same molecular formula and different property hence and d is correct option