It depends if you want to BUY the option or SELL the option.
As far as I know, you only need the cash necessary to buy the option. In the US, the exchange that handles most silver futures/options liquidity is the CME. An option is thus a right to buy/sell a futures contract. One futures contract represents 5000 Troy ounces and its price is quoted in cents. Let's say that Silver right now is quoted at 17. One futures contract then represent 17 * 5000 /100 USD = 850 USD.