In nature, carbon occurs in various forms, but the purest form of carbon is found in option 1, carbon black. Carbon black is a form of amorphous carbon that is derived from the incomplete combustion of hydrocarbons. It is composed of elemental carbon in the form of fine particles and does not contain any impurities.

Option 2, graphite, is another form of carbon that is naturally occurring, but it is not as pure as carbon black. Graphite is composed of carbon atoms arranged in a layered structure, and it often contains impurities such as clay, shale, or other minerals.

Option 3, diamond, is also a pure form of carbon, but it is less common in nature compared to carbon black and graphite. Diamonds are formed under high pressure and temperature deep within the Earth`s mantle. They are prized for their clarity and purity.

Option 4, coal, is a fossil fuel composed of decomposed plant matter, and it contains other elements besides carbon, such as sulfur and nitrogen. Coal is not considered the purest form of carbon in nature.

Therefore, the correct answer is option 1, carbon black, as it represents the purest form of carbon in nature.