"Tension stress of a fault" occurs when "slabs of rock" are being "pulled apart from each other".
Explanation:
When two equal forces that are acting in opposite directions are 'applied' on an object, the object is subjected to tension.
Same is the case with the slabs of rocks that are being pulled apart from each other. Thus, the force that is applied on the rocks before a fault occurs is a tensional force.
This force is quickly released after the fault has occurred.