Which of these would NOT weaken a causal conclusion?

Providing indisputable evidence of the cause strengthens the causal conclusion rather than weakening it. When robust evidence is presented that supports the existence of a causal relationship, it lends credibility to the argument that one factor is responsible for the effect in question. This type of evidence would serve to reinforce the notion that the cause is reliably connected to the effect, thereby solidifying the causal link rather than undermining it.

In contrast, showing that alternate causes exist, proving unobservability of the effect, or demonstrating that the cause occurs without the effect would all challenge the established causal relationship. Each of these approaches introduces doubt about whether the initial cause truly leads to the effect, which directly weakens the argument's validity.