Final answer:
The profit is the premium of $10 kept by the seller, as the call option expires worthless when the underlying asset's price ($80) is below the strike price ($100). Therefore, the correct option is C.
Step-by-step explanation:
The profit at maturity of a short call option that was sold for $10, with a strike price of $100, when the underlying asset price is $80 at expiry, can be calculated as follows:
The option will expire worthless since the underlying price ($80) is less than the strike price ($100). The option seller gets to keep the premium of $10 as profit. There's no obligation for the option seller to sell the underlying asset because it has not reached the strike price, hence no additional cost incurred.
Therefore, the answer is C. 10.