Selenium is the group 16 element that has 8 allotropic forms. Let's break down the answer and explain it in detail.

Allotropy refers to the existence of an element in two or more different physical forms, known as allotropes, in the same state. These allotropes have different molecular structures and properties.

Sulphur, oxygen, selenium, and polonium are all group 16 elements, also known as the chalcogens. Group 16 elements have six valence electrons and tend to form compounds with a -2 oxidation state.

Here's why selenium is the correct answer:

1. Sulphur:
Sulphur is well-known for its allotropes. It has several different forms, such as rhombic sulphur, monoclinic sulphur, and plastic sulphur. However, it does not have 8 allotropic forms, so it is not the correct answer.

2. Oxygen:
Oxygen is another group 16 element, but it primarily exists as a diatomic molecule (O2) in its most stable form. While it can form other compounds like ozone (O3), it does not have 8 allotropic forms, so it is not the correct answer.

3. Selenium:
Selenium is a semi-metal that has 8 known allotropic forms. These forms include gray selenium, red selenium, black selenium, vitreous selenium, crystalline selenium, metallic selenium, fibrous selenium, and amorphous selenium. Each form has a different molecular structure and physical properties.

4. Polonium:
Polonium is a radioactive element and is the heaviest of the group 16 elements. It is primarily known for its alpha particle emission and radioactive decay. However, it does not have 8 allotropic forms, so it is not the correct answer.

In conclusion, among the group 16 elements, selenium is the element that has 8 allotropic forms. These forms exhibit different molecular structures and properties, making selenium a fascinating element to study.