Which of the following is all organic?

The pulp is considered all organic because it is primarily composed of soft connective tissue and contains a high proportion of cells, fibers, and extracellular matrix, which are all organic in nature. The pulp serves several critical functions within a tooth, including housing nerves and blood vessels, contributing to tooth vitality, and playing a role in the tooth's response to stimuli and repair processes.

In contrast, other options contain significant inorganic components. Cementum, while largely organic, also contains inorganic materials, predominantly hydroxyapatite, which contribute to its structure. Enamel is the hardest and most mineralized substance in the body, with a composition that is around 96% inorganic minerals, making it predominantly inorganic. Dentin, like cementum, contains both organic and inorganic materials, with approximately 70% of its composition being inorganic, primarily hydroxyapatite, and 30% organic, mainly collagen.

Thus, the pulp's composition as entirely organic differentiates it from the other structures involved in the tooth's anatomy.