in starting mission 6 or 7 im supposed to go to the rally point to get my C4 to blow up the two bunkers. When i reach the rally point i press x to re-supply but i don't see the C4 in the supply options, i'm stuck.

Its been a while since I played but if I recall you dont actually carry the C4 with you, I 'think' you approach the item to be destroyed and are prompted to push Y (for xbox, unsure for other platforms) and in doing so you set up the explosion and run lol

As I say its been a while since I played but I dont remember ever having the C4 in my equipment.

Hope this helps