Explanation:
The marginal cost means that the cost has increased, mean it'll change, which means that the quantity of the product being produced has increase by one or more.
on the other hand, Marginal rain is where the revenue has also increased by one or more. so the additional revenue on the product means that the good will be equal to the sell price that the firm charges for the buyer.
Therefore, it'll be option 3 (I think)
Hope this helps!! have a nice day :)