The hydrodynamic theory states that a stimulus at the outer aspect of dentin causes fluid movement within the dentinal tubules. the hydrodynamic theory is the currently accepted explanation for transmission of stimuli from the outer surface of dentin to the pulp.

The hydrodynamic theory, the most widely accepted theory, indicates that when an appropriate stimulus (temperature, physical, or osmotic change) is applied to dentine, a change in the rate of fluid movement within the dentinal tubules occurs, creating a pressure change in the dentine and triggering a response in the pulp nerves, ultimately causing pain. The theory claims that tubules are open between dentine surface which is exposed to the environment and pulp.

Explanation:

Dentin hypersensitivity (DH, DHS) is dental pain which is sharp in character and of short duration, arising from exposed dentin surfaces in response to stimuli, typically thermal, evaporative, tactile, osmotic, chemical or electrical could be to air, cold, sweet, acidic or hot foods ,and which cannot be ascribed to any other dental disease but  cavities, cracked teeth, gum recession, enamel and root erosion all cause dentin to be exposed.

The dentinal fluid is the lymph or fluid of dentin, which appears on the surface of freshly cut dentin, especially in young teeth; it is a transudate of extracellular fluid, mainly cytoplasm of odontoblastic processes, from the dental pulp via the dentinal tubules.  

A nociceptor ("pain receptor") is a sensory neuron that responds to damaging or potentially damaging stimuli by sending “possible threat” signals to the spinal cord and the brain.

There are three major classes of nociceptors in the skin: Aδ mechanosensitive nociceptors, Aδ mechanothermal nociceptors, and polymodal nociceptors, the latter being specifically associated with C fibers.

The hydrodynamic theory, first proposed by M. Brännström in 1966, suggested that dentine hypersensitivity or pain sensation from teeth is due to the abrupt movement of fluid within the dentinal tubules in response to cold and heat (thermal), osmotic (e.g. eating sugary foods), mechanical (e.g. chewing) and evaporative stimuli (e.g. air blasts). It assumes that the is attributed to the stimulation of nociceptive mechanoreceptors (nociceptors) as a consequence of dentinal fluid flow (DFF) within DMTs.

the correct answer, according to e2020, is the second choice there is an occluded front in region a and a cold front in region b.