Introduction:
Tar is a thick, black, sticky substance that is commonly used in various industries such as construction, roofing, and road paving. It is a by-product that is obtained during the production of different materials. In this case, the correct answer is option 'D', which means that tar is a by-product of wood, petroleum, and coal. Let's explore each of these sources in detail.

Wood:
- Tar can be obtained from wood through a process called destructive distillation or pyrolysis.
- During this process, wood is heated in the absence of oxygen, resulting in the breakdown of its organic components.
- The volatile components, including tar, are collected and further processed to obtain the final product.

Petroleum:
- Tar can also be derived from petroleum, which is a naturally occurring fossil fuel.
- During the refining of petroleum, a heavy residue called bitumen is obtained.
- Bitumen is a thick, black substance that is commonly referred to as tar.
- It is used in various applications such as road construction and waterproofing.

Coal:
- Coal tar is another type of tar that is specifically obtained from coal.
- It is a by-product of the carbonization or destructive distillation of coal.
- When coal is heated in the absence of air, it undergoes a chemical transformation, resulting in the production of coal tar.
- Coal tar has various industrial applications, including in the manufacturing of roofing materials, paints, and coatings.

Conclusion:
Tar is a versatile substance that is obtained as a by-product from different sources such as wood, petroleum, and coal. It is widely used in industries due to its adhesive and waterproofing properties. The correct answer to the given question is option 'D', which includes all three sources of tar.