"Assuming the very thing he sets out to prove" usually refers to the improper use of an example, or circular reasoning. In this case, Lance doesn't set out to prove his conclusion. His only support is "experience."
I'll create a scenario in which A could be correct.
Lance: Experience teaches us that every general rule has at least one exception. For example, mice are generally smaller than horses, but someday we will find a horse-sized mouse.
Here, Lance attempts to prove his conclusion with an example, but the example requires the truth of the conclusion. It proves nothing. If Frank called him out on this, then A would be correct.
However, this is not how the actual argument proceeds. The real problem is the contradiction of Lance creating a general rule about general rules.
