The correct answer is option D: Radioactive substances.

Explanation:

The Stockholm Convention on Persistent Organic Pollutants (POPs) is a global treaty established in 2001 to protect human health and the environment from chemicals that are persistent, bioaccumulative, and toxic. It aims to eliminate or restrict the production, use, and release of these hazardous substances.

The Stockholm Convention initially identified 12 POPs, which are divided into three categories: pesticides, industrial chemicals, and unintentional by-products. These categories are defined as follows:

a) Pesticides: Pesticides are chemicals used to control pests, such as insects, weeds, and fungi. They are commonly used in agriculture, public health, and household settings. The three POPs in this category are:

- Aldrin
- Chlordane
- Dieldrin

b) Industrial chemicals: Industrial chemicals are substances used in various industrial processes, including manufacturing, production, and maintenance. The eight POPs in this category are:

- Polychlorinated biphenyls (PCBs)
- Hexachlorobenzene (HCB)
- DDT (dichlorodiphenyltrichloroethane)
- Heptachlor
- Mirex
- Toxaphene
- Endrin
- Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDFs)

c) By-products: By-products are unintentionally produced during various industrial processes, such as combustion and chemical reactions. The one POP in this category is:

- Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDFs)

d) Radioactive substances: Radioactive substances, such as isotopes of elements like uranium and plutonium, are not included in the list of POPs recognized under the Stockholm Convention. These substances have different properties and pose unique risks to human health and the environment compared to the 12 initial POPs.

In conclusion, while pesticides, industrial chemicals, and by-products are categories of the 12 initial POPs recognized under the Stockholm Convention, radioactive substances are not included in this list.