I don't know why there is not valid answer here, cause anda 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, anda 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", kismis, raisin (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 anda got the point)
