Fraenger's assertion that the artist Hieronymus Bosch belonged to the Brethren of the Free Spirit, a nonmainstream religious group, is unlikely to be correct.
Conclusion (rebuttal)
F's assertion that HB belonged to the Brethren of the Free Spirit is unlikely to be correct.
Rephrase: HB probably didn't belong to the Brethren of the Free Spirit.
Evidence
There's evidence that HB was a member of a mainstream church and no evidence he was a member of the Brethren.
Evaluate
Whenever we do Main Conclusion or Role, we remind ourselves that the Conclusion is typically - the first sentence and/or - some rebuttal or pivot idea
Here the first sentence was a rebuttal. The author then makes a concession towards her opponent in the 2nd sentence before introducing her supporting idea in the 3rd sentence.
Goal
We're being asked about the role of the final sentence, which is the author's evidence.
It has two claims, connected by "and" (claims connected by "and" are almost always supporting ideas).
So we're being asked about one of the two premises.