Final answer:
The false reasoning being used in the argument is 'begging the question', also known as circular reasoning. The person is claiming cause B without providing sufficient evidence and shifting the burden of proof onto you. Emphasize the need for compelling evidence before accepting any particular cause.
Step-by-step explanation:
The official definition of the false reasoning being used by the person you are arguing with is called 'begging the question'. It occurs when someone assumes the truth of the conclusion they are trying to prove or assumes the truth of a contentious claim in their argument. This fallacy is also known as circular reasoning.
In your case, the person is claiming that the change in outcome X is due to cause B without providing sufficient evidence. They are also shifting the burden of proof onto you to prove that the effect is not due to B before accepting cause A. However, the burden of proof is always on the person making the claim. You can counter their argument by pointing out the lack of compelling evidence supporting cause B and emphasizing the need for sufficient evidence before accepting any particular cause.