The writer of a put option has the obligation to buy the underlying security at the option's strike price if the option is exercised. This is because when an investor writes (sells) a put option, they are granting the buyer the right to sell a security at a predetermined price (the strike price). If the buyer exercises the put option, the writer is obligated to purchase the security at the agreed-upon strike price, regardless of the current market price. This could lead to a loss for the writer if the market price is lower than the strike price at exercise, but the writer receives a premium upfront that can offset some of this potential loss.

• Choice B is incorrect. The obligation to sell the underlying security at the strike price describes a call option writer, not a put option writer. In a put option, the writer's obligation is to buy, not sell.

• Choice C is incorrect. This describes the right, not obligation, to buy, which is the position of a call option buyer (holder), not a writer.

• Choice D is incorrect. This describes the right, not obligation, to sell, which is the position of a put option buyer (holder), not a writer.

Key concept: Buyers of options hold rights (not obligations), while writers (sellers) of options hold obligations (not rights).