A call option gives you the right but not the obligation to buy...

A call option gives you the right but not the obligation to buy the share at the strike price on or sometimes before the expiry date. You must check the terms of the option you are buying. (go back to the CRC announcement about the options on their website or the ASX site)

If the share price is above the strike price you will want to exercise your right to buy the share with the call option as it gives you a lower price for the share.

If the share price is below the strike price you are not obliged to exercise it. Of course you would not want to exercise it as this would mean you would pay the higher strike price for the share. It would be cheaper to buy the share direct on the market.

From what you say you want to pay 13c for a call now. When the option expires you will have to pay another 20c (if the strike is 20c) to buy a share exercised by the option. Shares on the market closed at 23c yesterday. You need to decide whether you want to pay 13c now and 20c later or just 23c now for the share. Its a bit like hire purchase.

I hope this helps
Serrata