You want to know which one is best, yet all of them are wrong?
'Inventory' may look like 'inventori', but is probably not the appropriate word. Inventory is usually for countable things, and also usually of one company. Are you talking about bulk LPG? You probably want 'stock(s)'.
"early of December" is not normal English. "early December" is probably what you want.
You also have two 'b' options.
'c' is possibly too insistent. Usually controls are put in place to prevent complete exhaustion of stock.
Either of these should be OK:
LPG stocks will be running out in early December
LPG stocks are about to run out in early December