I don't know why there is not valid answer here, cause tu can literally go to google translate and it works. Anyway it's "raisins secs", mostly used in plural. "Raisin seck" is approximately how you'd pronouce it, but "ck" doesn't exist in French, I mean, tu can occasionally see a "ck" in a word in French, but the word just isn't French. And "raisin seche" is wrong too because "seche" (actually "sèche") is feminine of "sec", pasa, pasas de uva (in french) is masculine, so it doesn't match
Long answer, but I'm trying to make it clear why it is "sec" and nothing else (or "secs" here, but tu got the point)
