The premium on a call option (C) is affected by three factors:
Spot price relative to the strike price (S – X): The higher the spot rate relative to the strike price, the higher the option price will be.
Length of time before expiration (T): The longer the time to expiration, the higher the option price will be.
Potential variability of currency (σ): The greater the variability of the currency, the higher the probability that the spot rate can rise above the strike price.