Which purpose does a relevant detail server

It evaluates facts and evidence
It supports the central idea of the text
It undermines the effectiveness of an argument
It identifies ideas not related to the topic

All of the above are purposes of a relevant detail in a text.

Which one is the most reasonable answer

The most reasonable answer is that a relevant detail serves to support the central idea of the text. While the other purposes listed are also valid, the primary function of a relevant detail is to provide evidence or examples that bolster or reinforce the main point or argument being made.