Bicep.
Six pack and the Abs are the same thing i.e. your abdominal muscles (2 packs and 4 packs if they exist would be partially developed abs, the whole abs pack including upper and lower would total to 6 and hence 6 pack) ..... From the wrist to your elbow its your forearm (both sides), the upper muscle running from your elbow to your shoulder muscle is your bicep, the lower is your tricep, then you have front deltoids (shoulder), lats, rear deltoids, the muscle connecting your deltoids and neck are traps .... and then you have upper back, lower back and lats ...... the lower body muscles are thighs and calves.