The answer is B. The spreading tectonic boundary causes fissures and faults in the crust that could extend close to the mantle. The volcanic activity shows an active region which results to heat from the mantle coming up and heating the water in the crust into steam.
The condition of spreading form of tectonic is all because of the faults into the crusts that would all the way be extending into the mantle. This shows that in this region it would be resulting into the condition of heat from the side of mantle.
