if benar means correct and kah means it is wouldn't this translate to is it correct?
kah is indicating a question "Right?" so you you could also say "Is that right?"
Benarkah = Betulkah?
banar is true
betul is good
Really? = sungguh?
Or in informal way we normally say "beneran?" It's the same as "benarkah?"
Serious? = serius? Seriusan?
Why is "Truly?" not accepted?
it's more synonymous to "very" rather than "right/true" in this context.
I found this explanation
Is "Right?" ok?