Answer:
a mediating factor
Step-by-step explanation:
Mediator is the factor that plays an intermediate role between two distinct parts connecting them. That is, the mediating factors are the "path" that leads one to an effect. In the case of the above question, Alexis's blood pressure is rising due to the stress he is experiencing. In this case, stress can be considered a mediating factor because it is the path leading to increased blood pressure.
Stress is responsible, it is the factor that is causing blood pressure to rise, so stress is the mediating factor.