The strike price. As the strike price decreases, call options become more valuable and put options become less valuable. The time to expiration. Put and call options become more valuable as the time to expiration increases. The volatility of the stock price.
Volatility is the relative fluctuation of the underlying stock price.
Put and call options become more valuable as the volatility increases. The risk-free interest rate. While the investor's carrying cost increases with an increase in the risk-free interest rate, the expected growth rate in the underlying stock price tends to dominate this effect. As the risk-free interest rate increases, the price of call options increases and the price of put options decreases. The dividends expected during the life of the option.