While it is simple to find if the letter "A" is in the string "A, B, C, max, one", there are nuances to it.
=COUNTMATCHES(B2,$A4)>0
This will look for the letter "A" anywhere in the string, case insensitive. There are two of them, the single letter "A" and the "a" in "max". Either one will return TRUE. This is probably not the answer you want.
=IFERROR(FIND($A4,B2)>0,FALSE)
This will look for "A", case sensitive. If cell B2 had "Alpha" as an option, it would return true even if there was not a standalone "A" in the list. Also probably not the answer you want.
=COUNTMATCHES(B2,REGEX("(^|, )"&$A4&"(, |$)",FALSE))>0
This complicated looking thing looks for "A" as being the entire string, "A, " at the beginning of the string, ", A, " somewhere in the middle, or ", A" at the end. It is case sensitive but can be made case sensitive. It requires each option in the list to all be followed by a comma and a space, except for the last one. An extraneous comma or misplaced or missing space could cause a problem. An option in the list can have a space in it like "max audio" and you can search for that term.