Which strategy below would a speculator implement to profit on future volatility of the underlying asset, without taking a stand on which direction the spot price will move?

a.) Buy both an at-the-money call option and an at-the-money put option

b.) Buy an at-the-money call option and write an at-the-money put option

c.) Write a put option with strike price K_1S

d.) Buy a call option with strike price K_1S