Directions (1-6): In the following questions, a sentence is divided into four parts. Choose the part of the sentence which may contain a grammatical error in it. If all the parts of the sentence are grammatically correct and contextually meaningful, choose option “no error” as your answer choice.

After finishing the (A)/ marathon, she treated himself (B)/ to a delicious meal (C)/ to celebrate her accomplishment. (D)

The correct option is B.

Detailed Explanation:
Let us analyze the sentence step-by-step to locate the grammatical error:
1. Part (A): "After finishing the" is grammatically correct and functions as the beginning of an introductory adverbial phrase containing a gerund ("finishing").
2. Part (B): "marathon, she treated himself" contains a error in pronoun agreement. The subject of the main clause is "she", which is a third-person singular feminine pronoun. The reflexive pronoun referring back to this subject must agree in gender and number. Therefore, the masculine reflexive pronoun "himself" must be replaced with the feminine reflexive pronoun "herself".
3. Part (C): "to a delicious meal" is grammatically correct and correctly follows the idiomatic expression "to treat oneself to [something]".
4. Part (D): "to celebrate her accomplishment." is grammatically correct and uses the appropriate feminine possessive adjective "her" to refer to the subject "she".

Thus, the corrected sentence should read: "After finishing the marathon, she treated herself to a delicious meal to celebrate her accomplishment."