Select the most appropriate option to fill in the blank.

When I solved the sum ______ I was very happy.

myself

The most appropriate option to fill in the blank is myself.

The word "myself" is a reflexive pronoun that refers to the speaker or writer. In this context, the speaker is saying that they were happy when they solved the sum themselves.

The other options are not as appropriate in this context.

• Itself refers to a thing. It is not grammatically correct to use the pronoun "itself" to refer to a person.
• Oneself is a reflexive pronoun that can be used to refer to any person. However, it is more formal than "myself" and is not commonly used in everyday speech.
• Herself refers to a female person. It is not grammatically correct to use the pronoun "herself" to refer to a male person.