Take the W. Even if your school has a "freshman forgiveness" policy or something like that where they'll replace your grade if you retake it, you still have to report the grade in your TMDSAS/AMCAS, and a D in a science can really suck. My advice is to take the W, but retake Gen Chem this summer and make an A (even if it means taking it at another college). Last year, I was failing O Chem 2 at SMU, so I dropped with a W, but I immediately retook it at UTD in the summer and made an A. A few months ago, I talked about my case with UTSW's admissions director and he said that I made a smart move in retaking the class immediately and making the A because it indicates that I'm intellectually capable, I just had some personal problems at the time. Hopefully you're making A's in your other classes; dropping can give you a 4.0 semester. 🙂