A bull spread is a type of options strategy that is used when the investor expects a moderate rise in the price of the underlying asset. This strategy is created by buying an option with a lower strike price and simultaneously selling an option with a higher strike price. In the context of this question, Hassan can create a bull spread by buying the put option with a strike price of USD 20 (which costs USD 2) and selling the put option with a strike price of USD 25 (which costs USD 2.5). The lower strike price put option (USD 20) is bought because Hassan expects the price of the underlying asset to increase. On the other hand, the higher strike price put option (USD 25) is sold to offset the cost of buying the lower strike price put option. This strategy allows Hassan to profit from a moderate increase in the price of the underlying asset while limiting his potential loss to the net premium paid for the options (i.e., the cost of the bought option minus the income from the sold option).

Choice B is incorrect. Buying the option with a higher strike price and simultaneously selling the put option with a lower strike price would not create a bull spread. This strategy would actually result in a bear spread, as it profits when the asset's price decreases.

Choice C is incorrect. Buying both options, regardless of their strike prices, does not create a bull spread either. This strategy is known as straddle or strangle depending on whether the strike prices are the same or different respectively, which aims to profit from large movements in either direction and not specifically from an increase in asset's price.

Choice D is incorrect. As explained above, there are specific strategies to establish a bull spread.