Arc's own codebase doesn't implement comments of any sort, but it relies on Racket's line comments, so those at least are reliable across Arc implementations.
; A multi-line comment (nestable):
#|foo bar
baz|#
; A commented-out s-expression:
#;(foo bar
baz)
; A commented-out s-expression which happens to be a string:
#;"foo bar
baz"
; A commented-out s-expression which happens to be a symbol:
#;|foo bar
baz|
; A shebang comment
#! foo bar \
baz